[libc++] Check _LIBCPP_USE_CLOCK_GETTIME before using clock_gettime
The clock_gettime function is available when _POSIX_TIMERS is defined.
We check for this and set _LIBCPP_USE_CLOCK_GETTIME accordingly since
59b3102739c. But check for _LIBCPP_USE_CLOCK_GETTIME was removed in
babd3aefc91. As a result, code is now trying to use clock_gettime even
on platform where it is not available and it is causing build failure with newlib
This patch restores the checks to fix this.