[SDL] decoupling all bundled X code

[Ryan C. Gordon]

> All X 7.x branch will be modular, so it would be better if SDL make
> use of this modularity now. IMHO it is generally better idea to use
> system libraries than bundled, as this helps to avoid conflicts.

There are a lot of systems that don't use x.org at all, or use pre-7.0 
builds, so we're going to have to consider a non-modular X11 for a long 
long time yet.

This doesn't generally affect SDL, though...it's all the same X11 
libraries once they get onto a system.

--ryan.