glvnd_pthread.h and glvnd_pthread.c now declare and define a GLVNDPthreadFuncs
struct, instead of having a separate declaration in libGLX and libGLdispatch.

This mostly just simplifies initialization a bit, but it will also make it
easier to have common code that uses pthreads functions.

Reviewed-by: Andy Ritger <aritger@nvidia.com>