SharedForward: Cast away const to call execvp on MinGW 64-bit

Change-Id: Ia74a754b34971e71d5352f48839abc8e9a313682

SharedForward.h.in

@@ -512,7 +512,7 @@ static void kwsys_shared_forward_execvp(const char* cmd,
   /* Invoke the child process.  */
 #if defined(_MSC_VER)
   _execvp(cmd, argv);
-#elif defined(__MINGW32__)
+#elif defined(__MINGW32__) && !defined(__MINGW64__)
   execvp(cmd, argv);
 #else
   execvp(cmd, (char* const*)argv);