[CM] --with-x11 switch

[Bill Schottstaedt]

I'd like to remove this configure switch from libxm/Snd -- would that
inconvenience anyone?  It would mean that a few X functions would not
be directly accessible in the gtk version of Snd, but surely no one should be
depending on that sort of hackery!

(I think the gtk project is working on a Mac-native version;
that's how I interpret some changes they've made; if this becomes an option,
it will make Snd much easier to build and use on a Mac).