diff --git a/lib/System/Unix/Unix.h b/lib/System/Unix/Unix.h
index 71445094afd..6dc75545b66 100644
--- a/lib/System/Unix/Unix.h
+++ b/lib/System/Unix/Unix.h
@@ -11,24 +11,78 @@
 //
 //===----------------------------------------------------------------------===//
 
+#ifndef LLVM_SYSTEM_UNIX_UNIX_H
+#define LLVM_SYSTEM_UNIX_UNIX_H
+
 //===----------------------------------------------------------------------===//
 //=== WARNING: Implementation here must contain only generic UNIX code that
 //===          is guaranteed to work on all UNIX variants.
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Config/config.h"     // Get autoconf configuration settings
-#include <unistd.h>
 #include <cstdlib>
 #include <cstdio>
 #include <cstring>
 #include <cerrno>
-#include <sys/types.h>
-#include <sys/param.h>
-#include <assert.h>
 #include <string>
 #include <algorithm>
 
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+
+#ifdef HAVE_SYS_TYPES_H
+#include <sys/types.h>
+#endif
+
+#ifdef HAVE_SYS_PARAM_H
+#include <sys/param.h>
+#endif 
+
+#ifdef HAVE_ASSERT_H
+#include <assert.h>
+#endif
+
+#ifdef TIME_WITH_SYS_TIME
+# include <sys/time.h>
+# include <time.h>
+#else
+# ifdef HAVE_SYS_TIME_H
+#  include <sys/time.h>
+# else
+#  include <time.h>
+# endif
+#endif
+
+#ifdef HAVE_SYS_WAIT_H
+# include <sys/wait.h>
+#endif
+
+#ifndef WEXITSTATUS
+# define WEXITSTATUS(stat_val) ((unsigned)(stat_val) >> 8)
+#endif
+
+#ifndef WIFEXITED
+# define WIFEXITED(stat_val) (((stat_val) & 255) == 0)
+#endif
+
 inline void ThrowErrno(const std::string& prefix) {
     char buffer[MAXPATHLEN];
-    throw prefix + ": " + strerror(errno);
+#ifdef HAVE_STRERROR_R
+    // strerror_r is thread-safe.
+    strerror_r(errno,buffer,MAXPATHLEN-1);
+#elif HAVE_STRERROR
+    // Copy the thread un-safe result of strerror into
+    // the buffer as fast as possible to minimize impact
+    // of collision of strerror in multiple threads.
+    strncpy(buffer,strerror(errno),MAXPATHLEN-1);
+    buffer[MAXPATHLEN-1] = 0;
+#else
+    // Strange that this system doesn't even have strerror
+    // but, oh well, just use a generic message
+    sprintf(buffer, "Error #%d", errno);
+#endif
+    throw prefix + ": " + buffer;
 }
+
+#endif