Windows support by Ruben Van Boxem.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@142235 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/thread b/include/thread
index ce3ca49..4366c41 100644
--- a/include/thread
+++ b/include/thread
@@ -100,7 +100,9 @@
 #endif
 #include <pthread.h>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 #define __STDCPP_THREADS__ __cplusplus