external dependencies

[Matthias Clasen <mclasen redhat com>]

The topic came up earlier, and I think there was a general consensus
that it is a good idea to freeze the versions of external dependencies,
and use tarball modules for them in the gnome-2.18 moduleset in jhbuild.

I see that we already do that for hal, with the hal-0-5-7 id. 
fontconfig 2.4.0 has just been released, so I'd propose that we do the
same for fontconfig now, and then follow with dbus, as soon as 1.0 is
released (which should be soon now).

There is a slight problem with doing this version freezing by changing
the ids, since it messes up dependencies (unless we add some from of
"provides" to jhbuild modules) - e.g. the hal-0-5-7 modules requires
"dbus", so if we freeze dbus as "dbus-1-0", the "dbus" dependency will
not be satisfied by it.

Matthias