argp revisited

[Tom Tromey <tromey creche cygnus com>]

I got some email back from Miles Bader.  The "argp" library is part of
GNU libc (>= version 2.1).

So I think we should just use it.  We can extract it for distribution
in Gnome, and arrange for it not to be built on machines that already
have the right glibc.

The only issue is getting Gtk option parsing to play well with
whatever we do.  But we can probably find a compromise that will work
well.

Tom