glx: Refactor the configure options for glx implementation choice (v3)

glx: Refactor the configure options for glx implementation choice (v3)

Instead of cascading support for various different implementations of
GLX, all three options are now specified through the --enable-glx
option:

  --enable-glx=dri          : Enable the DRI-based GLX
  --enable-glx=xlib         : Enable the classic Xlib-based GLX
  --enable-glx=gallium-xlib : Enable the gallium Xlib-based GLX
  --enable-glx[=yes]        : Defaults to dri if DRI is enabled, else
                              gallium-xlib if gallium is enabled, else
                              xlib

This removes the --enable-xlib-glx option and fixes a bug in which both
the classic xlib-glx and gallium xlib-glx implementations were getting
built causing different versioned and conflicting libGL libraries to be
installed.

v2: Changes from various review feedback from Emil:
  a) Fixed typos
  b) Corrected help docs for new option
  c) Added appropriate a-b and r-b tags in commit msg
  d) Fixed various GLX related dependency checks.
v3: Rebased to current master and added changelog in commit msg

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94086

Acked-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>