Since 7f2a6f17fc ("xdg-desktop-portal-gtk: Enable all default
portals unconditionally"), building upower 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, and it's consistent with the package's
"withIntrospection" argument (though not "useIMobileDevice").