nls: Do not update po files on first make invocation.

nls: Do not update po files on first make invocation.

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>