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/__bit_reference b/include/__bit_reference
index 53d3c86..fa381bb 100644
--- a/include/__bit_reference
+++ b/include/__bit_reference
@@ -14,7 +14,9 @@
 #include <__config>
 #include <algorithm>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD