Windows support by Ruben Van Boxem.

llvm-svn: 142235
diff --git a/libcxx/include/__bit_reference b/libcxx/include/__bit_reference
index 53d3c86..fa381bb 100644
--- a/libcxx/include/__bit_reference
+++ b/libcxx/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