New example 'hello-c++20'.
* gettext-tools/examples/hello-c++20: New directory, based on
gettext-tools/examples/hello-c++.
* gettext-tools/examples/Makefile.am (EXAMPLESFILES, EXAMPLESDIRS): Update.
* gettext-tools/examples/po/Makefile.am (POTFILES, SMALLPOTS,
SMALLPOFILES_FOR_lang): Update.
(hello-c++20.pot, hello-c++20/po/$(LL).po): New rules.
* gettext-tools/examples/README: Update.
* gettext-tools/examples/check-examples (func_check_autoclean_all,
func_check_distclean_all, func_check_maintainerclean_all,
func_check_maintainerclean_vpath_all, func_check_dist_all,
func_check_dist_vpath_all, func_check_install_all, func_check_uninstall_all,
func_check_distcheck_all, func_check_all): Handle this directory as well.
* NEWS: Mention the new example.
