autopoint: Reject AM_GNU_GETTEXT without 'external' for versions >= 0.20.
* gettext-tools/misc/autopoint.in: Fail if AM_GNU_GETTEXT without 'external' is
specified with a gettext version >= 0.20.
* gettext-tools/tests/autopoint-2: Update. Add two more scenarios.
* gettext-tools/tests/autopoint-3: Don't test AM_GNU_GETTEXT([use-libtool]) nor
--with-included-gettext. Don't use libtoolize.
