toplevel configure.in: simplify tests

[DJ Delorie]

> Hmm...  I think we're in bug-fix only mode in GCC mainline, so I'd
> approve this only to the gcc 3.4 small improvements branch.  But then,
> this would get GCC mainline out of sync with src, so DJ might be
> annoyed because his auto sync would stop working.  DJ, what do yo
> think?

There was a change just yesterday to gcc's toplevel files, so they're
already out of sync.  That change was described as a convenience
change, so it seems like the rules are flexible anyway.

It's three weeks until the 3.3 branch, unless it's delayed.  IMHO This
patch is harmless in the current tree, because we no longer support
cygnus-style configure anyway, so I would have no problem with it
going in.  But waiting a few weeks doesn't sound unreasonable either.