Windows support by Ruben Van Boxem.

llvm-svn: 142235
diff --git a/libcxx/include/ciso646 b/libcxx/include/ciso646
index 38b3e94..b2efc72 100644
--- a/libcxx/include/ciso646
+++ b/libcxx/include/ciso646
@@ -18,6 +18,8 @@
 
 #include <__config>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 #endif  // _LIBCPP_CISO646