java: Create reproducible .jar files if the 'jar' utility supports it.
Suggested by Bernhard M. Wiedemann <bwiedemann@suse.de> in
<https://lists.gnu.org/archive/html/bug-gettext/2024-07/msg00020.html>.
* build-aux/jar-cf: New file.
* Makefile.am (EXTRA_DIST): Add it.
* gettext-runtime/intl-java/Makefile.am (libintl.jar): Use jar-cf.
* gettext-tools/src/Makefile.am (gettext.jar): Likewise.
