Windows support by Ruben Van Boxem.

llvm-svn: 142235
diff --git a/libcxx/include/forward_list b/libcxx/include/forward_list
index 0dd6bb2..61fe290 100644
--- a/libcxx/include/forward_list
+++ b/libcxx/include/forward_list
@@ -174,7 +174,9 @@
 #include <iterator>
 #include <algorithm>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD