This makes it possible to build “gnunet-gtk” with the latest version of
GTK+.  I chose to disable the three tests, because they require an X
server with libGL support for software rendering.  I couldn’t get this
to work even after adding “xorg-server” and “mesa”, and starting Xvfb
before tests.

Patches for “gnunet-gtk” follow later (they also require changes to
our “gnunet” package).

~~ Ricardo