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.

configure

@@ -659,6 +659,7 @@ TCL_CONFIG_SH
 TCLSH
 XGETTEXT
 MSGMERGE
+MSGFMT_FLAGS
 MSGFMT
 HAVE_POSIX_SIGNALS
 LDAP_LIBS_BE
@@ -29346,6 +29347,19 @@ done
 $as_echo "$as_me: error: msgfmt is required for NLS" >&2;}
    { (exit 1); exit 1; }; }
   fi
+  { $as_echo "$as_me:$LINENO: checking for msgfmt flags" >&5
+$as_echo_n "checking for msgfmt flags... " >&6; }
+if test "${pgac_cv_msgfmt_flags+set}" = set; then
+  $as_echo_n "(cached) " >&6
+else
+  if test x"$MSGFMT" != x"" && "$MSGFMT" --version 2>&1 | grep "GNU" >/dev/null; then
+    pgac_cv_msgfmt_flags=-c
+fi
+fi
+{ $as_echo "$as_me:$LINENO: result: $pgac_cv_msgfmt_flags" >&5
+$as_echo "$pgac_cv_msgfmt_flags" >&6; }
+  MSGFMT_FLAGS=$pgac_cv_msgfmt_flags
+
   for ac_prog in msgmerge
 do
   # Extract the first word of "$ac_prog", so it can be a program name with args.