bug#57346: undeclared identifier 'CLOCK_MONOTONIC'

[Gregory Heytings]

> > Right, the internet says OS X doesn't have clock_gettime.
> >
> > Does gnulib provide it?
>
> Not AFAICT.

It should be easy to emulate it with what Mac OS X/macOS provides, 
however:

int
clock_gettime (clockid_t ignored /* assuming CLOCK_MONOTONIC */,
               struct timespec *tp)
{
  clock_serv_t cs;
  mach_timespec_t mts;
  kern_return_t ret;
  host_get_clock_service (mach_host_self (), SYSTEM_CLOCK, &cs);
  ret = clock_get_time (cs, &mts);
  mach_port_deallocate(mach_task_self (), cs);
  if (ret) return ret;
  tp->tv_sec = mts.tv_sec;
  tp->tv_nsec = mts.tv_nsec;
}