clock_gettime(CLOCK_REALTIME, &ts) is equivalent to timespec_get(&ts,
TIME_UTC) in C11.
It has the advantage that avoids the following building error when building
with macOS:
warning: 'timespec_get' is only available on macOS 10.15 or newer
[-Wunguarded-availability-new]
In 2012, with the release of OS X 10.8 Mountain Lion, the name of the
system was shortened from Mac OS X to OS X. In 2016, with the release
of macOS 10.12 Sierra, the name was changed from OS X to macOS to
streamline it with the branding of Apple's other primary operating
systems: iOS, watchOS, and tvOS.
Reference: https://en.wikipedia.org/wiki/MacOS
