Adding Xvfb to the vax build

[David Brownlee <abs%absd.org@localhost>]

        I think its safe to say that adding Xvfb to the X11 build
        makes a very small difference - its lost in the noise when
        crossbuilding on an AMD box here:

            Build without X11: 1 hour 0 minutes 21 seconds
            2899.95s user 643.88s system 97% cpu 1:00:28.14 total

            Build with X11, no Xvfb: 1 hour 20 minutes 09 seconds
            3855.29s user 843.70s system 97% cpu 1:20:14.59 total

            Build with X11, and Xvfb: 1 hour 20 minutes 14 seconds
            3795.18s user 839.59s system 96% cpu 1:20:23.99 total

        Note: I'm not saying it will only take a few more seconds
        when natively building on a vax, but the relative difference
        is very small, and if people want to exclude sections of
        the tree for a build they should add more USE_* or MK_*
        variables in a machine independent fashion so all ports
        can benefit.