Windows support by Ruben Van Boxem.

llvm-svn: 142235
diff --git a/libcxx/include/ios b/libcxx/include/ios
index e2f2b6f..7ea63a3 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -216,7 +216,9 @@
 #include <__locale>
 #include <system_error>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD