build: Turn update-po.sh into a Makefile rule
Suggested by Andreas Stricker in:
<https://lists.gnu.org/archive/html/bug-gettext/2014-04/msg00020.html>.
* configure.ac (maintainer-update-po): New recursive rule.
* update-po.sh: Remove.
* autogen.sh: Don't mention update-po.sh.
* Makefile.am (maintainer-update-po-local): New rule.
* gettext-runtime/Makefile.am (maintainer-update-po): New rule.
* gettext-runtime/man/Makefile.am (update-man1): Remove; add explicit
dependency on the executable to each .1 rule.
* gettext-tools/man/Makefile.am (update-man1): Remove; add explicit
dependency on the executable to each .1 rule.
