1) We use the preprocessor flag HAVE_EPOXY_GLX_H, but never
   set it.
2) WITH_GLX diverges with HAVE_EPOXY_EGL_H.
3) virgl_test_server needs to link with X11 when using a GLX
   backend.

To fix this, query epoxy for GLX support and allow the user to
disable it as well.

TEST=./autogen.sh --disable-egl