Resync from gnulib.

Resync from gnulib.

* cfg.mk (cvs_executable_files, cvs_files): Rewrite...
(fetch): ...into new target.
(executable-update): Delete, now that it is unused.
* maint.mk (update, local_updates, cvs_files, gnulib_repo)
(wget-update, cvs-update): Likewise.
* HACKING (Update the foreign files): Document new procedure.
* GNUmakefile: Resync from upstream, via new 'make fetch'.
* build-aux/config.guess: Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>