Why does thread support depend on GLX_DIRECT_RENDERING in libGL?

[Ian Romanick]

While working on bug #3024, I noticed something kind of odd.  Every 
place where there is code that depends on XTHREADS being defined, it 
also depends on GLX_DIRECT_RENDERING being defined.  The net result 
being that the only time you get thread safety in libGL (i.e., a GLX 
context per thread) is when GLX_DIRECT_RENDERING is defined.  This seems 
wrong.

Can any one explain why this is?  If not, I'm going to axe it as part of 
the bug fix.

https://bugs.freedesktop.org/show_bug.cgi?id=3024