Windows support by Ruben Van Boxem.

llvm-svn: 142235
diff --git a/libcxx/include/__hash_table b/libcxx/include/__hash_table
index f9578fa..4399caa 100644
--- a/libcxx/include/__hash_table
+++ b/libcxx/include/__hash_table
@@ -18,7 +18,9 @@
 #include <algorithm>
 #include <cmath>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD