082 Configure/build problem: we need to demand X11 when using GLX.

    As it is now, it's possible on certain platforms (at least
    experienced on Sun Solaris) that GLX linking is found to be ok by
    configure, even though X11 linking failed.

    This will then cause problems at compile or linkage later on, so
    we should detect this in the configure script and warn properly.

    20031028 mortene.

    UPDATE 20060208 mortene: larsa has been building on and off on
    Solaris lately, so presumably this error is no longer present?
