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.
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