In the POT files, talk about the "GNU gettext package".
Reported by Benno Schulenberg <coordinator@translationproject.org>.
* gettext-runtime/po/Makevars (XGETTEXT_OPTIONS): Add --package-name option.
* gettext-tools/po/Makevars (XGETTEXT_OPTIONS): Likewise.
* gettext-tools/examples/po/Makefile.am (XGETTEXT_OPTIONS): New variable.
