Based on a previous work by Tim Marston, this patch enables l18n support using the default automake/gettext layout.
The associated feature/l18n branch can be checked out here for a more convenient merge.

Following, there are decisions on optional actions that require maintainer's approval:

A) contact the translationproject.org to set up the program page and upload the pot file. I could then provide a French translation so that you can experiment the l10n workflow;
B) share the catalog with gengetopt users so that their translators won't have to translate strings contained in skeleton files by using gengetopt translations as a compendium. However, there are two ways of doing it:
1) waive your own copyright received from translators (by asking for the disclaimer) and put translations in the public domain; or,
2) generate a shared catalog in the public domain and keep gengetopt specific strings under your copyright.

Remaining work to achieve for:
B.1: add skeleton files to POTFILES to make sure every string in embedded even those not used by gengetopt itself. Add skeleton files to the dist files so they are found by xgettext in a dist build;
B.2: create a second pot file made of skeleton files then modify the build process to use associated translations as a compendium for the original pot file and then modify dist rules to package the shared catalog and certainly some more. It is quite painful to implement due to the complex interactions with autopoint generated scripts it would require (see branch feature/l18n-shared-catalog for a tentative).