xvfb not built on vax and x68k?

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

        x11/Xserver/Makefile.common and
        external/mit/xorg/server/xorg-server/Makefile.common

        specifically set XSERVER_XVFB=no for x68k and vax, but
        builds it for all other ports. This appears to have been
        set in the first version of x11/Xserver/Makefile.common.

        Does anyone have any idea why this is done (apart from
        possibly to save build time), and more to the point, any
        objections to making those two ports consistent with all
        the others so all NetBSD ports build the Xvfb virtual
        framebuffer X server?