[RFA] Update Darwin support in top-level configure.in

[DJ Delorie]

> I think this should be sent to GCC.  Nathanael is currently merging the 
> two top-level directories so the place to get this committed is in GCC.

Nathanael is doing the autoconfiscation.  The toplevel directories are
pretty much already merged, at least for the important bits we
maintain locally.  As far as I'm concerned, any top level patch can be
approved on either side, as long as it gets applied to both sides
(either by the submitter, or if they can't, by me).