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/__std_stream b/include/__std_stream
index 6ab0fd6..6df6556 100644
--- a/include/__std_stream
+++ b/include/__std_stream
@@ -17,7 +17,9 @@
 #include <__locale>
 #include <cstdio>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD