Make sure dep_egl is always a valid dependency object, even if not
found. Ensure it is not found when not wanted, to avoid linking when
found but not wanted.
Using a not-found dependency in Meson is defined to be a safe no-op, so
use that to simplify the backend dependencies.
libweston/meson.build already errors out if renderer-gl is enabled and
EGL is not found, so the same checks can be removed from the backends.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
915d8e8cdf