Otherwise, cups will be built without systemd support. The upstream
systemd service unit expects cups to have systemd support however. When
cups is built without systemd support, the daemon will fork to the
background. Consequently, systemd will think the service is done and
enter inactive state.
Along with 9de93be6cd448bdce64e7ef5cf80ff8d81592140 and
47330b1732b67e7d5fee283685b48fdcafdc91e6 this patch fixes the nixos
printer test timing out in `waitForUnit("cups.service")`; see
NixOS/nixpkgs#14748
7d32d92464