Try to prevent direct use of exit(), malloc(), etc

Use `#pragma GCC poison` (supported since GCC 3, maybe earlier) when
compiling with GCC to help prevent direct uses being introduced for
functions which DOH provides a wrapper for.

Source/Modules/main.cxx

@@ -1417,6 +1417,4 @@ static void SWIG_exit_handler(int status) {
       }
     }
   }
-
-  exit(status);
 }