Since 7f2a6f17fc ("xdg-desktop-portal-gtk: Enable all default
portals unconditionally"), building gnome-desktop 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.
6f700a0b61