gl: use dependency fallbacks to get at a possible gl-headers subproject

gl: use dependency fallbacks to get at a possible gl-headers subproject

We would make the subproject conditional on the wrap-mode=nodownload
however get_option('wrap-mode') or similar is not available from
meson.build files as the wrap-mode is meant to be used automatically.

Instead use the dependency fallback mechanism to get at the subproject
where possible i.e. when downloading is allows and only add the compat
includes when we have a valid internal dependency from the gl-headers
subproject.

https://bugzilla.gnome.org/show_bug.cgi?id=796534