Check for MinGW

Summary: Fix for: https://github.com/facebook/wdt/issues/203

Reviewed By: vitaut

Differential Revision: D24954835

fbshipit-source-id: 92f29f71cd6eef9bdcbe92ed1ecb8f1f5d24326e

folly/portability/Builtins.h

@@ -16,7 +16,7 @@
 
 #pragma once
 
-#if defined(_WIN32) && !defined(__clang__)
+#if defined(_WIN32) && !defined(__MINGW32__) && !defined(__clang__)
 #include <assert.h>
 #include <folly/Portability.h>
 #include <intrin.h>