Since 7f2a6f17fc ("xdg-desktop-portal-gtk: Enable all default
portals unconditionally"), building colord 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 "enableSystemd" for consistency with the
existing "enableDaemon" option.
As usual in Nixpkgs, the user is responsible for also overriding udev
to a non-systemd implementation if necessary.
4b2dbd4511