docs/autoconf: update glx driver / enable-debug text

With earlier commit we folded all the xlib handling in --enable-glx, but
we forgot to update the documentation.

Elaborate on --enable-debug and drop mentions about depenencies.

v2: Grammar - s|haven't|hasn't| (Eric)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
1 file changed