hurd configure update

[Roland McGrath]

I've added a new configure script and trivial shell script in the hurd/
subdir that give some formality to the simple copying needed for populating
a virgin cross compilation setup so you can build libc.  Somebody should
update docs and faqs to describe the bootstrap procedure as:

        1. install mach headers (unchanged)
        2. Run hurd/hurd/configure (works in a separate dir or the srcdir),
           giving --prefix=/usr/local/i686-gnu or whatever is appropriate
           for where you want /include/hurd headers installed under.
        3. Run ./install-headers (shell script configure creates)
        4. Now you can build libc with this cross compilation setup

With that done, I've put in Jeff's changes to the main configure script to
be compatible with newer Autoconf versions.  This breaks the old feature of
being able to configure with an incomplete cross compilation tree.  That
should not be required any longer because with the new procedure above,
you never configure the hurd until after you have libc built and installed.

I've also put the generated configure files in the repository, since we
have had consensus on the wisdom of that for a while now.