po, examples: Change .po -> .gmo rules to consider the newest changes to the POT...

po, examples: Change .po -> .gmo rules to consider the newest changes to the POT file.

Reported by Claude Paroz <claude@2xlibre.net>
in <https://savannah.gnu.org/bugs/?50910>.

* gettext-runtime/m4/po.m4 (AM_PO_SUBDIRS): Set MSGMERGE_FOR_MSGFMT_OPTION.
* gettext-runtime/po/Makefile.in.in (MSGMERGE): Use GNU msgmerge, even if not
first in $PATH.
(MSGMERGE_FOR_MSGFMT_OPTION): New variable.
(.po.gmo): Depend on the POT file. Use msgmerge on the fly, to take into
account the most recent POT file changes.
* gettext-tools/examples/hello-*/po/Makefile.am: Likewise, also for the .po.qm,
update-properties, update-classes targets.
* gettext-tools/examples/hello-objc-gnustep/po/GNUmakefile
(MSGMERGE_FOR_MSGFMT_OPTION): New variable.
($(STRINGSFILES)): Depend on the POT file. Use msgmerge on the fly, to take
into account the most recent POT file changes.