Windows support by Ruben Van Boxem.

llvm-svn: 142235
diff --git a/libcxx/include/ratio b/libcxx/include/ratio
index 9764014..4ef66c5 100644
--- a/libcxx/include/ratio
+++ b/libcxx/include/ratio
@@ -70,7 +70,9 @@
 #include <climits>
 #include <type_traits>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD