So that we can be started by systemd --user, instead of gnome-session.
There are three units:
- gnome-shell.service: Start gnome-shell itself.
- gnome-shell-x11.target, gnome-shell-wayland.target: Sync points for
units that need to care if x11 or wayland is in use.
gnome-settings-daemon will use these, for example.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/138
Meson is on track to replace autotools as the build system of choice,
so support it in addition to autotools. If all goes well, we'll
eventually be able to drop the latter ...
https://bugzilla.gnome.org/show_bug.cgi?id=783229