Include windows.h proxy header, instead of windows.h itself

Because the signal changes were in a different branch from the windows.h
related changes, the SEH handling code included the header directly.

Fixes #803

include/internal/catch_fatal_condition.hpp

@@ -23,11 +23,8 @@ namespace Catch {
 
 #if defined ( CATCH_PLATFORM_WINDOWS ) /////////////////////////////////////////
 
-#define NOMINMAX
-#define WIN32_LEAN_AND_MEAN
-#include <windows.h>
-#undef WIN32_LEAN_AND_MEAN
-#undef NOMINMAX
+
+#include "catch_windows_h_proxy.h"
 
 
 namespace Catch {