It's quite  complicated now: cmake WITH_SYSTEMD flag is also used to generate systemd units, tarantool-generator and other systemd related files.

I think that support for NOTIFY_SOCKET should not be systemd-related, as it's enabled in runtime by envar and is disabled otherwise.

-#if defined(WITH_SYSTEMD)

I don't think you should remove this ifdef at all. Simply enable
it on Mac OSX by default in cmake if it works there now.