po: Make it possible to override xgettext options from Makefile.in.in.
* gettext-runtime/po/Makefile.in.in ($(DOMAIN).pot-update): List the
XGETTEXT_OPTIONS and the XGETTEXT_EXTRA_OPTIONS after all other options.
* gettext-tools/examples/po/Makefile.am ($(DOMAIN).pot-update): Likewise.
* gettext-tools/examples/hello-*/po/Makefile.am ($(DOMAIN).pot-update):
Likewise.
* gettext-tools/examples/hello-objc-gnustep/po/GNUmakefile
($(DOMAIN).pot-update): Likewise.
* NEWS: Mention the change.
* gettext-tools/doc/gettext.texi (po/Makevars): Fix typo.
