The xclock_settime is a wrapper function on the clock_settime syscall
to be used in the test code.
It checks if the GLIBC_TEST_ALLOW_TIME_SETTING env variable is defined
in the environment in which test is executed. If it is not - the test
ends as unsupported. Otherwise, the clock-settime is executed and return
value is assessed.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
