Since 7f2a6f17fc ("xdg-desktop-portal-gtk: Enable all default
portals unconditionally"), building gnome-settings-daemon is necessary
to build xdg-desktop-portal-gtk, but it wasn't possible for systems
that can't use systemd.
I've called the option "withSystemd" as it's the most common name for
this kind of option in Nixpkgs.
A couple of backports are required for it to build. They can be
removed when we update to GNOME 47.
