common_quakedef.h (and thus all files)
This solves a build break when the OpenGL includes are not in /usr/include,
since the OpenGL include path is only used when compiling files that use
OpenGL.
We now use dlsym() to check for XMesaSetFXmode() at runtime.
If we find XMesaSetFXmode() all DGA code is disabled so it won't clash
with Glide (Glide on Voodoo 3 uses DGA internally...)
For XMESA we now check the MESA_GLX_FX environment variable at startup to
detect whether we are started in windowed or fullscreen mode, and then
set the cvar vid_glx_fullscreen to match that.
