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>
diff --git a/git-compat-util.h b/git-compat-util.h
index 9b495dc..c915752 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -389,8 +389,13 @@
 #define ST_CTIME_NSEC(st) 0
 #define ST_MTIME_NSEC(st) 0
 #else
+#ifdef USE_ST_TIMESPEC
+#define ST_CTIME_NSEC(st) ((unsigned int)((st).st_ctimespec.tv_nsec))
+#define ST_MTIME_NSEC(st) ((unsigned int)((st).st_mtimespec.tv_nsec))
+#else
 #define ST_CTIME_NSEC(st) ((unsigned int)((st).st_ctim.tv_nsec))
 #define ST_MTIME_NSEC(st) ((unsigned int)((st).st_mtim.tv_nsec))
 #endif
+#endif
 
 #endif