Summary:
- Mach doesn't support clock_getres(3) or clock_gettime(3). Provide our
own implementations by wrapping mach_timebase_info() and
mach_absolute_time().
- Detect availability of of -lrt at configure time and don't link with
it if it's not available. On Linux, this is what provides
clock_gettime(3) and friends.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Ubuntu/FC/Mac

Reviewed By: meyering@fb.com

FB internal diff: D999131