Windows support by Ruben Van Boxem.

llvm-svn: 142235
diff --git a/libcxx/include/unordered_map b/libcxx/include/unordered_map
index cb30fc8..56645e0 100644
--- a/libcxx/include/unordered_map
+++ b/libcxx/include/unordered_map
@@ -319,7 +319,9 @@
 #include <functional>
 #include <stdexcept>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD