Define _LIBCPP_HAS_TIMESPEC_GET for FreeBSD when appropriate

Authored by dim on Jan 29 2020, 11:00 PM.


Define _LIBCPP_HAS_TIMESPEC_GET for FreeBSD when appropriate

FreeBSD got timespec_get support somewhere in the 12.x timeframe, but
the C++ version check in its system headers was written incorrectly.
This has now been fixed for both FreeBSD 13 and 12.

Add checks for the corresponding __FreeBSD_version values, to define
_LIBCPP_HAS_TIMESPEC_GET when the function is supported.

Reviewers: emaste, EricWF, ldionne, mclow.lists

Reviewed By: ldionne

Subscribers: arichardson, krytarowski, christof, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D71522