po: Make "make maintainer-clean" erase the .pot file.

po: Make "make maintainer-clean" erase the .pot file.

Rationale:
<https://www.gnu.org/prep/standards/html_node/Standard-Targets.html>
<https://www.gnu.org/software/automake/manual/html_node/Clean.html>

* NEWS: Mention the change.
* gettext-runtime/po/Makefile.in.in (maintainer-clean): Remove the .pot file.
* gettext-tools/examples/hello-*/po/Makefile.am (MAINTAINERCLEANFILES): Add
the .pot file.
* gettext-tools/examples/po/Makefile.am (MAINTAINERCLEANFILES): Likewise.
* gettext-tools/examples/check-examples (func_check_maintainerclean,
func_check_maintainerclean_vpath): Verify the .pot file is removed after
'make maintainer-clean', except when using the older Makefile.in.in
infrastructure.