Remove maintainer-check target, fold into normal build

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.

config/programs.m4

@@ -197,6 +197,11 @@ AC_DEFUN([PGAC_CHECK_GETTEXT],
   if test -z "$MSGFMT"; then
     AC_MSG_ERROR([msgfmt is required for NLS])
   fi
+  AC_CACHE_CHECK([for msgfmt flags], pgac_cv_msgfmt_flags,
+[if test x"$MSGFMT" != x"" && "$MSGFMT" --version 2>&1 | grep "GNU" >/dev/null; then
+    pgac_cv_msgfmt_flags=-c
+fi])
+  AC_SUBST(MSGFMT_FLAGS, $pgac_cv_msgfmt_flags)
   AC_CHECK_PROGS(MSGMERGE, msgmerge)
   AC_CHECK_PROGS(XGETTEXT, xgettext)
 ])# PGAC_CHECK_GETTEXT