The otherwise recommended clock_get_time(SYSTEM_TIME) is about 40x
slower than mach_absolute_time() even after minimizing overhead by
retrieving the port right only once per process - see
https://github.com/nigoroll/varnish-cache/tree/macos_SYSTEM_CLOCK

So we revert to mach_absolute_time() which needs some scaling but
otherwise does the job. It is reported to halt on iOS devices when
sleeping, but even if anyone came up with the idea to sell varnish
over the app store, sleeping would probably need to be avoided in the
first place and otherwise the right way would probably be to adjust
the offset using pre/post sleep callbacks.

As MacOS should have been the last platform using the real clock
fallback for VTIM_mono, we now bail out at compile-time for any
platform we have overlooked.