meson: move gl pkgconfig generation out of glx

Because the same generation logic is required by xlib glx and
gallium-xlib glx, it makes sense to pull it out.

v2: - Ensure that libgl is defined before trying to generate a pkgconfig
      file with it.

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