meson: build glx

meson: build glx

This gets GLX and the loader building. The resulting GLX and i965 have
been tested on piglit and seem to work fine. This patch leaves a lot of
todo's in it's wake, GLX is quite complicated, and the build options
involved are many, and the goal at the moment is to get dri and gallium
drivers building.

v2: - fix typo "vaule" -> "value"
    - put the not on the correct element of the conditional
    - Put correct description of dri3 option in this patch not the next
      one (Eric A)
    - fix non glvnd version (Eric A)
    - build glx tests
    - move loader include variables to this patch (Eric A)
v3: - set the version correctly for GL_LIB_NAME in libglx
v4: - set pkgconfig private fields

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>