We need to update the minimal gettext version to take advantage of new
features. Before this patch, the first make invocation would modify
po/guix and po/packages po files, and we advised to run `git checkout
po` to clean the changes.
* configure.ac (AM_GNU_GETTEXT_VERSION): Update to 0.19.1.
* po/guix/Makevars: Set PO_DEPENDS_ON_POT to no.
* po/packages/Makevars: Set PO_DEPENDS_ON_POT to no.
Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>