Hi Guix,

As you probably know, after the first make in a clean checkout of the
guix repository, po files are dirty and we advertise to run "git
checkout po" to get them clean again. This was caused by the gettext
generated makefile that wanted to build the mo (compiled po) files, and
these po files depend on the pot that needs to be generated.

In newer versions of gettext (at least 0.19.1 from 4 years ago), there
is an option, PO_DEPENDS_ON_POT that I set to "no", so this patch
updates the minimal gettext version to 0.19.1 and makes use of that
variable, that removes the dependency from the po files to the pot file.

This means that when make tries to generate the mo files, it can do so
immediately, without needing to generate the pot file nor update the po
files that depended on the pot.

As this patch updates the minimal requirement for gettext to 0.19.1,
I'm not sure what's the impact. This is a version from 4 years ago
(0.18.1 was from 6 years ago). We currently have gettext 0.20.1, and
debian jessie has 0.19.3, so I don't think this change will negatively
affect our users:

- developpers should already use guix environment guix which provides a
  good version.
- packagers of guix on other distros might have a harder time, but even
  debian seems to have a recent enough version

WDYT?