We require logind for the native backend, but the backend itself is
optional. However since commit 06c357d78, we will always throw an
error if neither logind nor elogind are available, even when the
backend is disabled.
As we still support "auto" - that is, whether the native backend is
enabled depends on whether its dependencies are available - the
easiest option is to make sure we always include either elogind or
libsystemd in the dependency check rather than erring out explicitly
if neither is found.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/96
82564772dc