Windows support by Ruben Van Boxem.

llvm-svn: 142235
diff --git a/libcxx/include/numeric b/libcxx/include/numeric
index 31493990..4ca62e8 100644
--- a/libcxx/include/numeric
+++ b/libcxx/include/numeric
@@ -60,7 +60,9 @@
 #include <__config>
 #include <iterator>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD