make maintainer-check was obscure and rarely called in practice, and
many breakages were missed.  Fold everything that make maintainer-check
used to do into the normal build.  Specifically:

- Call duplicate_oids when genbki.pl is called.

- Check for tabs in SGML files when the documentation is built.

- Run msgfmt with the -c option during the regular build.  Add an
  additional configure check to see whether we are using the GNU
  version.  (make maintainer-check probably used to fail with non-GNU
  msgfmt.)

Keep maintainer-check as around as phony target for the time being in
case anyone is calling it.  But it won't do anything anymore.