diff --git a/autoconf/configure.ac b/autoconf/configure.ac
index 7e399fcddd0..243b2ceec9f 100644
--- a/autoconf/configure.ac
+++ b/autoconf/configure.ac
@@ -115,81 +115,6 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([[#if ! __clang__
 AC_LANG_POP([C++])
 AC_MSG_RESULT([${llvm_cv_cxx_compiler}])
 
-dnl Check both GCC and Clang for sufficiently modern versions. These checks can
-dnl be bypassed by passing a flag if necessary on a platform.
-AC_ARG_ENABLE(compiler-version-checks,
-              AS_HELP_STRING([--enable-compiler-version-checks],
-                             [Check the version of the host compiler (default is YES)]),,
-                             enableval=default)
-case "$enableval" in
-  no)
-    ;;
-  yes|default)
-    AC_LANG_PUSH([C++])
-    case "$llvm_cv_cxx_compiler" in
-    clang)
-      AC_MSG_CHECKING([whether Clang is new enough])
-      AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
-#if __clang_major__ < 3 || (__clang_major__ == 3 && __clang_minor__ < 1)
-#error This version of Clang is too old to build LLVM
-#endif
-]])],
-          [AC_MSG_RESULT([yes])],
-          [AC_MSG_RESULT([no])
-           AC_MSG_ERROR([
-The selected Clang compiler is not new enough to build LLVM. Please upgrade to
-Clang 3.1. You may pass --disable-compiler-version-checks to configure to
-bypass these sanity checks.])])
-
-      dnl Note that libstdc++4.6 is known broken for C++11 builds. The errors
-      dnl are sometimes deeply confusing though. Here we test for an obvious
-      dnl incomplete feature in 4.6's standard library that was completed in
-      dnl 4.7's.
-      AC_MSG_CHECKING([whether Clang will select a modern C++ standard library])
-      llvm_cv_old_cxxflags="$CXXFLAGS"
-      CXXFLAGS="$CXXFLAGS -std=c++0x"
-      AC_LINK_IFELSE([AC_LANG_SOURCE([[
-#include <atomic>
-std::atomic<float> x(0.0f);
-int main() { return (float)x; }
-]])],
-          [AC_MSG_RESULT([yes])],
-          [AC_MSG_RESULT([no])
-           AC_MSG_ERROR([
-We detected a missing feature in the standard C++ library that was known to be
-missing in libstdc++4.6 and implemented in libstdc++4.7. There are numerous
-C++11 problems with 4.6's library, and we don't support GCCs or libstdc++ older
-than 4.7. You will need to update your system and ensure Clang uses the newer
-standard library.
-
-If this error is incorrect or you need to force things to work, you may pass
-'--disable-compiler-version-checks' to configure to bypass this test.])])
-      CXXFLAGS="$llvm_cv_old_cxxflags"
-      ;;
-    gcc)
-      AC_MSG_CHECKING([whether GCC is new enough])
-      AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
-#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 7)
-#error This version of GCC is too old to build LLVM
-#endif
-]])],
-          [AC_MSG_RESULT([yes])],
-          [AC_MSG_RESULT([no])
-           AC_MSG_ERROR([
-The selected GCC C++ compiler is not new enough to build LLVM. Please upgrade
-to GCC 4.7. You may pass --disable-compiler-version-checks to configure to
-bypass these sanity checks.])])
-      ;;
-    unknown)
-      ;;
-    esac
-    AC_LANG_POP([C++])
-    ;;
-  *)
-    AC_MSG_ERROR([Invalid setting for --enable-compiler-version-checks. Use "yes" or "no"])
-    ;;
-esac
-
 dnl Configure all of the projects present in our source tree. While we could
 dnl just AC_CONFIG_SUBDIRS on the set of directories in projects that have a
 dnl configure script, that usage of the AC_CONFIG_SUBDIRS macro is deprecated.
@@ -582,6 +507,83 @@ case "$enableval" in
   *) AC_MSG_ERROR([Invalid setting for --enable-libcpp. Use "yes" or "no"]) ;;
 esac
 
+dnl Check both GCC and Clang for sufficiently modern versions. These checks can
+dnl be bypassed by passing a flag if necessary on a platform. We have to do
+dnl these checks here so that we have the configuration of the standard C++
+dnl library finished.
+AC_ARG_ENABLE(compiler-version-checks,
+              AS_HELP_STRING([--enable-compiler-version-checks],
+                             [Check the version of the host compiler (default is YES)]),,
+                             enableval=default)
+case "$enableval" in
+  no)
+    ;;
+  yes|default)
+    AC_LANG_PUSH([C++])
+    case "$llvm_cv_cxx_compiler" in
+    clang)
+      AC_MSG_CHECKING([whether Clang is new enough])
+      AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
+#if __clang_major__ < 3 || (__clang_major__ == 3 && __clang_minor__ < 1)
+#error This version of Clang is too old to build LLVM
+#endif
+]])],
+          [AC_MSG_RESULT([yes])],
+          [AC_MSG_RESULT([no])
+           AC_MSG_ERROR([
+The selected Clang compiler is not new enough to build LLVM. Please upgrade to
+Clang 3.1. You may pass --disable-compiler-version-checks to configure to
+bypass these sanity checks.])])
+
+      dnl Note that libstdc++4.6 is known broken for C++11 builds. The errors
+      dnl are sometimes deeply confusing though. Here we test for an obvious
+      dnl incomplete feature in 4.6's standard library that was completed in
+      dnl 4.7's.
+      AC_MSG_CHECKING([whether Clang will select a modern C++ standard library])
+      llvm_cv_old_cxxflags="$CXXFLAGS"
+      CXXFLAGS="$CXXFLAGS -std=c++0x"
+      AC_LINK_IFELSE([AC_LANG_SOURCE([[
+#include <atomic>
+std::atomic<float> x(0.0f);
+int main() { return (float)x; }
+]])],
+          [AC_MSG_RESULT([yes])],
+          [AC_MSG_RESULT([no])
+           AC_MSG_ERROR([
+We detected a missing feature in the standard C++ library that was known to be
+missing in libstdc++4.6 and implemented in libstdc++4.7. There are numerous
+C++11 problems with 4.6's library, and we don't support GCCs or libstdc++ older
+than 4.7. You will need to update your system and ensure Clang uses the newer
+standard library.
+
+If this error is incorrect or you need to force things to work, you may pass
+'--disable-compiler-version-checks' to configure to bypass this test.])])
+      CXXFLAGS="$llvm_cv_old_cxxflags"
+      ;;
+    gcc)
+      AC_MSG_CHECKING([whether GCC is new enough])
+      AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
+#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 7)
+#error This version of GCC is too old to build LLVM
+#endif
+]])],
+          [AC_MSG_RESULT([yes])],
+          [AC_MSG_RESULT([no])
+           AC_MSG_ERROR([
+The selected GCC C++ compiler is not new enough to build LLVM. Please upgrade
+to GCC 4.7. You may pass --disable-compiler-version-checks to configure to
+bypass these sanity checks.])])
+      ;;
+    unknown)
+      ;;
+    esac
+    AC_LANG_POP([C++])
+    ;;
+  *)
+    AC_MSG_ERROR([Invalid setting for --enable-compiler-version-checks. Use "yes" or "no"])
+    ;;
+esac
+
 dnl --enable-cxx11 : check whether or not to use -std=c++11 on the command line
 AC_ARG_ENABLE(cxx11,
               AS_HELP_STRING([--enable-cxx11],
diff --git a/configure b/configure
index a1b63f88888..0f03eace1e5 100755
--- a/configure
+++ b/configure
@@ -1395,11 +1395,11 @@ if test -n "$ac_init_help"; then
 Optional Features:
   --disable-FEATURE       do not include FEATURE (same as --enable-FEATURE=no)
   --enable-FEATURE[=ARG]  include FEATURE [ARG=yes]
+  --enable-polly          Use polly if available (default is YES)
+  --enable-libcpp         Use libc++ if available (default is NO)
   --enable-compiler-version-checks
                           Check the version of the host compiler (default is
                           YES)
-  --enable-polly          Use polly if available (default is YES)
-  --enable-libcpp         Use libc++ if available (default is NO)
   --enable-cxx11          Use c++11 if available (default is NO)
   --enable-split-dwarf    Use split-dwarf if available (default is NO)
   --enable-clang-arcmt    Enable building of clang ARCMT (default is YES)
@@ -3702,264 +3702,6 @@ ac_compiler_gnu=$ac_cv_c_compiler_gnu
 { echo "$as_me:$LINENO: result: ${llvm_cv_cxx_compiler}" >&5
 echo "${ECHO_T}${llvm_cv_cxx_compiler}" >&6; }
 
-# Check whether --enable-compiler-version-checks was given.
-if test "${enable_compiler_version_checks+set}" = set; then
-  enableval=$enable_compiler_version_checks;
-else
-  enableval=default
-fi
-
-case "$enableval" in
-  no)
-    ;;
-  yes|default)
-    ac_ext=cpp
-ac_cpp='$CXXCPP $CPPFLAGS'
-ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
-ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
-ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
-
-    case "$llvm_cv_cxx_compiler" in
-    clang)
-      { echo "$as_me:$LINENO: checking whether Clang is new enough" >&5
-echo $ECHO_N "checking whether Clang is new enough... $ECHO_C" >&6; }
-      cat >conftest.$ac_ext <<_ACEOF
-/* confdefs.h.  */
-_ACEOF
-cat confdefs.h >>conftest.$ac_ext
-cat >>conftest.$ac_ext <<_ACEOF
-/* end confdefs.h.  */
-
-#if __clang_major__ < 3 || (__clang_major__ == 3 && __clang_minor__ < 1)
-#error This version of Clang is too old to build LLVM
-#endif
-
-_ACEOF
-rm -f conftest.$ac_objext
-if { (ac_try="$ac_compile"
-case "(($ac_try" in
-  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
-  *) ac_try_echo=$ac_try;;
-esac
-eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
-  (eval "$ac_compile") 2>conftest.er1
-  ac_status=$?
-  grep -v '^ *+' conftest.er1 >conftest.err
-  rm -f conftest.er1
-  cat conftest.err >&5
-  echo "$as_me:$LINENO: \$? = $ac_status" >&5
-  (exit $ac_status); } &&
-	 { ac_try='test -z "$ac_cxx_werror_flag" || test ! -s conftest.err'
-  { (case "(($ac_try" in
-  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
-  *) ac_try_echo=$ac_try;;
-esac
-eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
-  (eval "$ac_try") 2>&5
-  ac_status=$?
-  echo "$as_me:$LINENO: \$? = $ac_status" >&5
-  (exit $ac_status); }; } &&
-	 { ac_try='test -s conftest.$ac_objext'
-  { (case "(($ac_try" in
-  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
-  *) ac_try_echo=$ac_try;;
-esac
-eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
-  (eval "$ac_try") 2>&5
-  ac_status=$?
-  echo "$as_me:$LINENO: \$? = $ac_status" >&5
-  (exit $ac_status); }; }; then
-  { echo "$as_me:$LINENO: result: yes" >&5
-echo "${ECHO_T}yes" >&6; }
-else
-  echo "$as_me: failed program was:" >&5
-sed 's/^/| /' conftest.$ac_ext >&5
-
-	{ echo "$as_me:$LINENO: result: no" >&5
-echo "${ECHO_T}no" >&6; }
-           { { echo "$as_me:$LINENO: error:
-The selected Clang compiler is not new enough to build LLVM. Please upgrade to
-Clang 3.1. You may pass --disable-compiler-version-checks to configure to
-bypass these sanity checks." >&5
-echo "$as_me: error:
-The selected Clang compiler is not new enough to build LLVM. Please upgrade to
-Clang 3.1. You may pass --disable-compiler-version-checks to configure to
-bypass these sanity checks." >&2;}
-   { (exit 1); exit 1; }; }
-fi
-
-rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
-
-                              { echo "$as_me:$LINENO: checking whether Clang will select a modern C++ standard library" >&5
-echo $ECHO_N "checking whether Clang will select a modern C++ standard library... $ECHO_C" >&6; }
-      llvm_cv_old_cxxflags="$CXXFLAGS"
-      CXXFLAGS="$CXXFLAGS -std=c++0x"
-      cat >conftest.$ac_ext <<_ACEOF
-/* confdefs.h.  */
-_ACEOF
-cat confdefs.h >>conftest.$ac_ext
-cat >>conftest.$ac_ext <<_ACEOF
-/* end confdefs.h.  */
-
-#include <atomic>
-std::atomic<float> x(0.0f);
-int main() { return (float)x; }
-
-_ACEOF
-rm -f conftest.$ac_objext conftest$ac_exeext
-if { (ac_try="$ac_link"
-case "(($ac_try" in
-  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
-  *) ac_try_echo=$ac_try;;
-esac
-eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
-  (eval "$ac_link") 2>conftest.er1
-  ac_status=$?
-  grep -v '^ *+' conftest.er1 >conftest.err
-  rm -f conftest.er1
-  cat conftest.err >&5
-  echo "$as_me:$LINENO: \$? = $ac_status" >&5
-  (exit $ac_status); } &&
-	 { ac_try='test -z "$ac_cxx_werror_flag" || test ! -s conftest.err'
-  { (case "(($ac_try" in
-  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
-  *) ac_try_echo=$ac_try;;
-esac
-eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
-  (eval "$ac_try") 2>&5
-  ac_status=$?
-  echo "$as_me:$LINENO: \$? = $ac_status" >&5
-  (exit $ac_status); }; } &&
-	 { ac_try='test -s conftest$ac_exeext'
-  { (case "(($ac_try" in
-  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
-  *) ac_try_echo=$ac_try;;
-esac
-eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
-  (eval "$ac_try") 2>&5
-  ac_status=$?
-  echo "$as_me:$LINENO: \$? = $ac_status" >&5
-  (exit $ac_status); }; }; then
-  { echo "$as_me:$LINENO: result: yes" >&5
-echo "${ECHO_T}yes" >&6; }
-else
-  echo "$as_me: failed program was:" >&5
-sed 's/^/| /' conftest.$ac_ext >&5
-
-	{ echo "$as_me:$LINENO: result: no" >&5
-echo "${ECHO_T}no" >&6; }
-           { { echo "$as_me:$LINENO: error:
-We detected a missing feature in the standard C++ library that was known to be
-missing in libstdc++4.6 and implemented in libstdc++4.7. There are numerous
-C++11 problems with 4.6's library, and we don't support GCCs or libstdc++ older
-than 4.7. You will need to update your system and ensure Clang uses the newer
-standard library.
-
-If this error is incorrect or you need to force things to work, you may pass
-'--disable-compiler-version-checks' to configure to bypass this test." >&5
-echo "$as_me: error:
-We detected a missing feature in the standard C++ library that was known to be
-missing in libstdc++4.6 and implemented in libstdc++4.7. There are numerous
-C++11 problems with 4.6's library, and we don't support GCCs or libstdc++ older
-than 4.7. You will need to update your system and ensure Clang uses the newer
-standard library.
-
-If this error is incorrect or you need to force things to work, you may pass
-'--disable-compiler-version-checks' to configure to bypass this test." >&2;}
-   { (exit 1); exit 1; }; }
-fi
-
-rm -f core conftest.err conftest.$ac_objext \
-      conftest$ac_exeext conftest.$ac_ext
-      CXXFLAGS="$llvm_cv_old_cxxflags"
-      ;;
-    gcc)
-      { echo "$as_me:$LINENO: checking whether GCC is new enough" >&5
-echo $ECHO_N "checking whether GCC is new enough... $ECHO_C" >&6; }
-      cat >conftest.$ac_ext <<_ACEOF
-/* confdefs.h.  */
-_ACEOF
-cat confdefs.h >>conftest.$ac_ext
-cat >>conftest.$ac_ext <<_ACEOF
-/* end confdefs.h.  */
-
-#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 7)
-#error This version of GCC is too old to build LLVM
-#endif
-
-_ACEOF
-rm -f conftest.$ac_objext
-if { (ac_try="$ac_compile"
-case "(($ac_try" in
-  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
-  *) ac_try_echo=$ac_try;;
-esac
-eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
-  (eval "$ac_compile") 2>conftest.er1
-  ac_status=$?
-  grep -v '^ *+' conftest.er1 >conftest.err
-  rm -f conftest.er1
-  cat conftest.err >&5
-  echo "$as_me:$LINENO: \$? = $ac_status" >&5
-  (exit $ac_status); } &&
-	 { ac_try='test -z "$ac_cxx_werror_flag" || test ! -s conftest.err'
-  { (case "(($ac_try" in
-  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
-  *) ac_try_echo=$ac_try;;
-esac
-eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
-  (eval "$ac_try") 2>&5
-  ac_status=$?
-  echo "$as_me:$LINENO: \$? = $ac_status" >&5
-  (exit $ac_status); }; } &&
-	 { ac_try='test -s conftest.$ac_objext'
-  { (case "(($ac_try" in
-  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
-  *) ac_try_echo=$ac_try;;
-esac
-eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
-  (eval "$ac_try") 2>&5
-  ac_status=$?
-  echo "$as_me:$LINENO: \$? = $ac_status" >&5
-  (exit $ac_status); }; }; then
-  { echo "$as_me:$LINENO: result: yes" >&5
-echo "${ECHO_T}yes" >&6; }
-else
-  echo "$as_me: failed program was:" >&5
-sed 's/^/| /' conftest.$ac_ext >&5
-
-	{ echo "$as_me:$LINENO: result: no" >&5
-echo "${ECHO_T}no" >&6; }
-           { { echo "$as_me:$LINENO: error:
-The selected GCC C++ compiler is not new enough to build LLVM. Please upgrade
-to GCC 4.7. You may pass --disable-compiler-version-checks to configure to
-bypass these sanity checks." >&5
-echo "$as_me: error:
-The selected GCC C++ compiler is not new enough to build LLVM. Please upgrade
-to GCC 4.7. You may pass --disable-compiler-version-checks to configure to
-bypass these sanity checks." >&2;}
-   { (exit 1); exit 1; }; }
-fi
-
-rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
-      ;;
-    unknown)
-      ;;
-    esac
-    ac_ext=c
-ac_cpp='$CPP $CPPFLAGS'
-ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
-ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
-ac_compiler_gnu=$ac_cv_c_compiler_gnu
-
-    ;;
-  *)
-    { { echo "$as_me:$LINENO: error: Invalid setting for --enable-compiler-version-checks. Use \"yes\" or \"no\"" >&5
-echo "$as_me: error: Invalid setting for --enable-compiler-version-checks. Use \"yes\" or \"no\"" >&2;}
-   { (exit 1); exit 1; }; }
-    ;;
-esac
-
 
 
 if test -d ${srcdir}/projects/test-suite ; then
@@ -4790,6 +4532,264 @@ echo "$as_me: error: Invalid setting for --enable-libcpp. Use \"yes\" or \"no\""
    { (exit 1); exit 1; }; } ;;
 esac
 
+# Check whether --enable-compiler-version-checks was given.
+if test "${enable_compiler_version_checks+set}" = set; then
+  enableval=$enable_compiler_version_checks;
+else
+  enableval=default
+fi
+
+case "$enableval" in
+  no)
+    ;;
+  yes|default)
+    ac_ext=cpp
+ac_cpp='$CXXCPP $CPPFLAGS'
+ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
+
+    case "$llvm_cv_cxx_compiler" in
+    clang)
+      { echo "$as_me:$LINENO: checking whether Clang is new enough" >&5
+echo $ECHO_N "checking whether Clang is new enough... $ECHO_C" >&6; }
+      cat >conftest.$ac_ext <<_ACEOF
+/* confdefs.h.  */
+_ACEOF
+cat confdefs.h >>conftest.$ac_ext
+cat >>conftest.$ac_ext <<_ACEOF
+/* end confdefs.h.  */
+
+#if __clang_major__ < 3 || (__clang_major__ == 3 && __clang_minor__ < 1)
+#error This version of Clang is too old to build LLVM
+#endif
+
+_ACEOF
+rm -f conftest.$ac_objext
+if { (ac_try="$ac_compile"
+case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
+  (eval "$ac_compile") 2>conftest.er1
+  ac_status=$?
+  grep -v '^ *+' conftest.er1 >conftest.err
+  rm -f conftest.er1
+  cat conftest.err >&5
+  echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); } &&
+	 { ac_try='test -z "$ac_cxx_werror_flag" || test ! -s conftest.err'
+  { (case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
+  (eval "$ac_try") 2>&5
+  ac_status=$?
+  echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); }; } &&
+	 { ac_try='test -s conftest.$ac_objext'
+  { (case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
+  (eval "$ac_try") 2>&5
+  ac_status=$?
+  echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); }; }; then
+  { echo "$as_me:$LINENO: result: yes" >&5
+echo "${ECHO_T}yes" >&6; }
+else
+  echo "$as_me: failed program was:" >&5
+sed 's/^/| /' conftest.$ac_ext >&5
+
+	{ echo "$as_me:$LINENO: result: no" >&5
+echo "${ECHO_T}no" >&6; }
+           { { echo "$as_me:$LINENO: error:
+The selected Clang compiler is not new enough to build LLVM. Please upgrade to
+Clang 3.1. You may pass --disable-compiler-version-checks to configure to
+bypass these sanity checks." >&5
+echo "$as_me: error:
+The selected Clang compiler is not new enough to build LLVM. Please upgrade to
+Clang 3.1. You may pass --disable-compiler-version-checks to configure to
+bypass these sanity checks." >&2;}
+   { (exit 1); exit 1; }; }
+fi
+
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+
+                              { echo "$as_me:$LINENO: checking whether Clang will select a modern C++ standard library" >&5
+echo $ECHO_N "checking whether Clang will select a modern C++ standard library... $ECHO_C" >&6; }
+      llvm_cv_old_cxxflags="$CXXFLAGS"
+      CXXFLAGS="$CXXFLAGS -std=c++0x"
+      cat >conftest.$ac_ext <<_ACEOF
+/* confdefs.h.  */
+_ACEOF
+cat confdefs.h >>conftest.$ac_ext
+cat >>conftest.$ac_ext <<_ACEOF
+/* end confdefs.h.  */
+
+#include <atomic>
+std::atomic<float> x(0.0f);
+int main() { return (float)x; }
+
+_ACEOF
+rm -f conftest.$ac_objext conftest$ac_exeext
+if { (ac_try="$ac_link"
+case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
+  (eval "$ac_link") 2>conftest.er1
+  ac_status=$?
+  grep -v '^ *+' conftest.er1 >conftest.err
+  rm -f conftest.er1
+  cat conftest.err >&5
+  echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); } &&
+	 { ac_try='test -z "$ac_cxx_werror_flag" || test ! -s conftest.err'
+  { (case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
+  (eval "$ac_try") 2>&5
+  ac_status=$?
+  echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); }; } &&
+	 { ac_try='test -s conftest$ac_exeext'
+  { (case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
+  (eval "$ac_try") 2>&5
+  ac_status=$?
+  echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); }; }; then
+  { echo "$as_me:$LINENO: result: yes" >&5
+echo "${ECHO_T}yes" >&6; }
+else
+  echo "$as_me: failed program was:" >&5
+sed 's/^/| /' conftest.$ac_ext >&5
+
+	{ echo "$as_me:$LINENO: result: no" >&5
+echo "${ECHO_T}no" >&6; }
+           { { echo "$as_me:$LINENO: error:
+We detected a missing feature in the standard C++ library that was known to be
+missing in libstdc++4.6 and implemented in libstdc++4.7. There are numerous
+C++11 problems with 4.6's library, and we don't support GCCs or libstdc++ older
+than 4.7. You will need to update your system and ensure Clang uses the newer
+standard library.
+
+If this error is incorrect or you need to force things to work, you may pass
+'--disable-compiler-version-checks' to configure to bypass this test." >&5
+echo "$as_me: error:
+We detected a missing feature in the standard C++ library that was known to be
+missing in libstdc++4.6 and implemented in libstdc++4.7. There are numerous
+C++11 problems with 4.6's library, and we don't support GCCs or libstdc++ older
+than 4.7. You will need to update your system and ensure Clang uses the newer
+standard library.
+
+If this error is incorrect or you need to force things to work, you may pass
+'--disable-compiler-version-checks' to configure to bypass this test." >&2;}
+   { (exit 1); exit 1; }; }
+fi
+
+rm -f core conftest.err conftest.$ac_objext \
+      conftest$ac_exeext conftest.$ac_ext
+      CXXFLAGS="$llvm_cv_old_cxxflags"
+      ;;
+    gcc)
+      { echo "$as_me:$LINENO: checking whether GCC is new enough" >&5
+echo $ECHO_N "checking whether GCC is new enough... $ECHO_C" >&6; }
+      cat >conftest.$ac_ext <<_ACEOF
+/* confdefs.h.  */
+_ACEOF
+cat confdefs.h >>conftest.$ac_ext
+cat >>conftest.$ac_ext <<_ACEOF
+/* end confdefs.h.  */
+
+#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 7)
+#error This version of GCC is too old to build LLVM
+#endif
+
+_ACEOF
+rm -f conftest.$ac_objext
+if { (ac_try="$ac_compile"
+case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
+  (eval "$ac_compile") 2>conftest.er1
+  ac_status=$?
+  grep -v '^ *+' conftest.er1 >conftest.err
+  rm -f conftest.er1
+  cat conftest.err >&5
+  echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); } &&
+	 { ac_try='test -z "$ac_cxx_werror_flag" || test ! -s conftest.err'
+  { (case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
+  (eval "$ac_try") 2>&5
+  ac_status=$?
+  echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); }; } &&
+	 { ac_try='test -s conftest.$ac_objext'
+  { (case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval "echo \"\$as_me:$LINENO: $ac_try_echo\"") >&5
+  (eval "$ac_try") 2>&5
+  ac_status=$?
+  echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); }; }; then
+  { echo "$as_me:$LINENO: result: yes" >&5
+echo "${ECHO_T}yes" >&6; }
+else
+  echo "$as_me: failed program was:" >&5
+sed 's/^/| /' conftest.$ac_ext >&5
+
+	{ echo "$as_me:$LINENO: result: no" >&5
+echo "${ECHO_T}no" >&6; }
+           { { echo "$as_me:$LINENO: error:
+The selected GCC C++ compiler is not new enough to build LLVM. Please upgrade
+to GCC 4.7. You may pass --disable-compiler-version-checks to configure to
+bypass these sanity checks." >&5
+echo "$as_me: error:
+The selected GCC C++ compiler is not new enough to build LLVM. Please upgrade
+to GCC 4.7. You may pass --disable-compiler-version-checks to configure to
+bypass these sanity checks." >&2;}
+   { (exit 1); exit 1; }; }
+fi
+
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+      ;;
+    unknown)
+      ;;
+    esac
+    ac_ext=c
+ac_cpp='$CPP $CPPFLAGS'
+ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_c_compiler_gnu
+
+    ;;
+  *)
+    { { echo "$as_me:$LINENO: error: Invalid setting for --enable-compiler-version-checks. Use \"yes\" or \"no\"" >&5
+echo "$as_me: error: Invalid setting for --enable-compiler-version-checks. Use \"yes\" or \"no\"" >&2;}
+   { (exit 1); exit 1; }; }
+    ;;
+esac
+
 # Check whether --enable-cxx11 was given.
 if test "${enable_cxx11+set}" = set; then
   enableval=$enable_cxx11;