Windows support by Ruben Van Boxem.

llvm-svn: 142235
diff --git a/libcxx/include/stack b/libcxx/include/stack
index 3d72f96..59906bd 100644
--- a/libcxx/include/stack
+++ b/libcxx/include/stack
@@ -85,7 +85,9 @@
 #include <__config>
 #include <deque>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD