Bug 1504022 - Restore mingw overrides for __try/__except (backout of …

…1498676) r=bobowen

mingw-clang, when using SEH exceptions, compile these fine but don't unwind
them properly. When using sj/lj exceptions it can't compile them at all.

mozilla-config.h.in

@@ -45,6 +45,17 @@
  */
 #if defined(CHROMIUM_SANDBOX_BUILD) && defined(XP_WIN)
 #include "base/win/sdkdecls.h"
+
+#ifdef __MINGW32__
+/*
+ * MinGW doesn't support __try / __except. There are a few mechanisms available
+ * to hack around it and pseudo-support it, but these are untested in Firefox.
+ * What is tested (and works) is replacing them with if(true) and else.
+ */
+#define __try if(true)
+#define __except(x) else
+
+#endif /* __MINGW32__ */
 #endif /* defined(CHROMIUM_SANDBOX_BUILD) && defined(XP_WIN) */
 
 #endif /* MOZILLA_CONFIG_H */