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/__mutex_base b/include/__mutex_base
index 9e472fc..5568765 100644
--- a/include/__mutex_base
+++ b/include/__mutex_base
@@ -16,7 +16,9 @@
 #include <system_error>
 #include <pthread.h>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 #ifdef _LIBCPP_SHARED_LOCK