[tlbuild] patches from Debian

[Karl Berry]

    Two changes: Add the a `:' which makes sure that if TMPDIR is set but empty,

Sigh.  I suppose it doesn't hurt, but this is far from the only place
that uses ${VAR-val}.  I don't much want to change it in just one place,
and I also don't want to change it everywhere at this point.  I also
don't see that it's a problem in practice, since it's been this way
forever.

    use of mktemp without any args.

That part seems fine, except that you have a lot of X's.  I don't know
if there are restrictions, but it wouldn't surprise me.

Maybe best to leave it alone at this point?

karl