[tlbuild] new build of bibtex/bibtex8: failure when aux file is not in cwd

[Karl Berry]

    how to handle this?

I think that setting TEXMFOUTPUT or openout_any is the best way, despite
the burden on users.

The alternative I see would be to change bibtex so that output is
written in cwd by default, like TeX.  That would have been the right
thing to do 25 years ago, but at this point, I suspect such a change
would cause far more problems than the absolute-path-name failture in
the first place.

By the way, we ultimately want makeindex to behave the same way.  I'm
not sure if it does right now.