__STDC_NO_ATOMICS doesn't seem to exist. In fact the only compiler
I've found that sets any of those is msvc, but it sets
__STDC_NO_ATOMICS__, not __STDC_NO_ATOMICS.
__STDC_NO_ATOMICS__ is the one documented by C11 standard.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6848>
On Windows and macOS always use the proper monotonic clock, including
for gst_util_get_timestamp(), and initialize its state only once.
Also on macOS use clock_gettime() for the realtime clock, if available
instead of always falling back to GLib.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4658>
