Hackathon: fix build errors on the "master" branch.

[Ricardo Wurmus]

Hi Guix,

for the past weeks the "master" branch has been in pretty poor state
according to ci.guix.gnu.org.  It keeps hovering at around 56% progress,
which is a far cry from the 80+% we used to have in the old days.

A lot of packages are in fact broken.  Many more are marked as broken
due to a bug affecting ci.guix.gnu.org.

It would be a good idea to do a little spring cleaning and go through
the dashboard for the master branch and work on fixing builds that are
currently broken on x86_64-linux.  (Other architectures are in worse
state, but fixing x86_64-linux may be something we can actually
accomplish.)

What do you think?

-- 
Ricardo