Windows support by Ruben Van Boxem.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@142235 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/cwchar b/include/cwchar
index 0a67fe8..6fe4852 100644
--- a/include/cwchar
+++ b/include/cwchar
@@ -110,7 +110,9 @@
 #include <support/win32/support.h> // pull in *swprintf defines
 #endif // _WIN32
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD