po: In the Makefiles, enable --quiet by default when possible.

po: In the Makefiles, enable --quiet by default when possible.

This is possible because msgmerge became faster
- through an index-based fuzzy search (msgl-fsearch.c) in 2006,
- through OpenMP parallelization in 2006,
- through processor speedups over the years.
The only case where msgmerge is still slow is when a compendium is in use.

Suggested by Marcus Müller <marcus_savannah@hostalia.de> in
<https://lists.gnu.org/archive/html/bug-gnulib/2023-03/msg00034.html>.

* gettext-runtime/po/Makefile.in.in ($(POFILES), update-po): If msgmerge has
version ≥ 0.11, pass option --quiet.
* gettext-runtime/po/Makevars (MSGMERGE_OPTIONS): Don't suggest --quiet any
more.
* gettext-tools/po/Makevars (MSGMERGE_OPTIONS): Likewise.
* gettext-tools/examples/hello-*/po/Makevars (MSGMERGE_OPTIONS): Likewise.
* gettext-tools/examples/hello-*/po/Makefile.am ($(POFILES), update-po): If
msgmerge has version ≥ 0.11, pass option --quiet.
(MSGMERGE_OPTIONS): Don't suggest --quiet any more.
* gettext-tools/examples/hello-objc-gnustep/po/GNUmakefile: Likewise.
* gettext-tools/examples/po/Makefile.am ($(POFILES), update-po): Pass option
--quiet to msgmerge.
(MSGMERGE_OPTIONS): Don't suggest --quiet any more.