[tlbuild] On the warnings emitted by gcc4 on Cygwin

[Karl Berry]

    Are we sure that the above is the right code for Cygwin?

I am sure of nothing about Cygwin.  I have never run it.  All I have
done is install patches provided by other people.  So as far as I am
concerned, if you and/or Rodrigo and/or anyone can send me a reasonable
patch that eliminates the warning(s) or otherwise improves things, I
will be happy to install it.

Thanks,
karl