Create USE_ST_TIMESPEC and turn it on for Darwin

Create USE_ST_TIMESPEC and turn it on for Darwin

Not all OSes use st_ctim and st_mtim in their struct stat.  In
particular, it appears that OS X uses st_*timespec instead.  So add a
Makefile variable and #define called USE_ST_TIMESPEC to switch the
USE_NSEC defines to use st_*timespec.

This also turns it on by default for OS X (Darwin) machines.  Likely
this is a sane default for other BSD kernels as well, but I don't have
any to test that assumption on.

Signed-off-by: Brian Gernhardt <benji@silverinsanity.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>