Lay the groundwork for my upcoming ilist sentinel shrinking patch

by defining a LLVM_COMPACT_SENTINELS symbol to 0 or 1 in config.h.

I'm asking for 3 favors:
- may an autofoo expert look at this and suggest improvements?
- may a cmake expert suggest analogous functionality for config.h?
- may somebody with the right autofoo mix regenerate configure? (mine is too new)

Thanks!
--This line, and those below, will be ignored--

M    configure.ac


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78628 91177308-0d34-0410-b5e6-96231b3b80d8

autoconf/configure.ac

@@ -312,6 +312,17 @@ else
   AC_SUBST(DISABLE_ASSERTIONS,[[DISABLE_ASSERTIONS=1]])
 fi
 
+dnl LLVM_COMPACT_SENTINELS : can be used to shrink ilist's end iterators and disable certain checks on them:
+if test ${ENABLE_OPTIMIZED},${DISABLE_ASSERTIONS} = "ENABLE_OPTIMIZED=1,DISABLE_ASSERTIONS=1" ; then
+  AC_SUBST(LLVM_COMPACT_SENTINELS,[1])
+else
+  AC_SUBST(LLVM_COMPACT_SENTINELS,[0])
+fi
+
+AC_DEFINE_UNQUOTED([LLVM_COMPACT_SENTINELS],$LLVM_COMPACT_SENTINELS,
+                   [Define to 1 for ilist sentinel compaction])
+
+
 dnl --enable-expensive-checks : check whether they want to turn on expensive debug checks:
 AC_ARG_ENABLE(expensive-checks,AS_HELP_STRING(
   --enable-expensive-checks,[Compile with expensive debug checks enabled (default is NO)]),, enableval="no")