[WIP]: localize guix.texi

[Julien Lepiller]

Hi,

here is my first attempt at localizing guix.texi. This patch adds
guix.fr.texi, a localized version of the manual. It uses po4a to
generate the .po file and use that same file to generate the localized
manual.

I had to include the guix.fr.texi in the commit because it is necessary
that it exists in order to add it to info_TEXINFOS. This means that
this version of guix now requires po4a and will always build the
localized manuals in every available language. I don't really like that
though.

I had to add a rule in Makefile.am without which guix.fr.info wouldn't
be built. Similar rules are required for generating the html and pdf
versions.

thoughts?