<unistd.h> smoke test
unistd_available:

# At the time of writing, we don't actually use CLOCK_REALTIME.
# But it's the only clock that's guaranteed by POSIX.
clock_gettime(CLOCK_REALTIME) smoke test
clock_gettime_available:CLOCK_REALTIME

# Used for mbedtls_ms_time() on platforms where we don't think
# CLOCK_BOOTTIME is available.
clock_gettime(CLOCK_MONOTONIC) smoke test
clock_gettime_available:CLOCK_MONOTONIC
