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>

Makefile

@@ -126,6 +126,9 @@ all::
 # randomly break unless your underlying filesystem supports those sub-second
 # times (my ext3 doesn't).
 #
+# Define USE_ST_TIMESPEC if your "struct stat" uses "st_ctimespec" instead of
+# "st_ctim"
+#
 # Define NO_NSEC if your "struct stat" does not have "st_ctim.tv_nsec"
 # available.  This automatically turns USE_NSEC off.
 #
@@ -663,6 +666,7 @@ ifeq ($(uname_S),Darwin)
 	endif
 	NO_MEMMEM = YesPlease
 	THREADED_DELTA_SEARCH = YesPlease
+	USE_ST_TIMESPEC = YesPlease
 endif
 ifeq ($(uname_S),SunOS)
 	NEEDS_SOCKET = YesPlease
@@ -928,6 +932,9 @@ endif
 ifdef NO_ST_BLOCKS_IN_STRUCT_STAT
 	BASIC_CFLAGS += -DNO_ST_BLOCKS_IN_STRUCT_STAT
 endif
+ifdef USE_ST_TIMESPEC
+	BASIC_CFLAGS += -DUSE_ST_TIMESPEC
+endif
 ifdef NO_NSEC
 	BASIC_CFLAGS += -DNO_NSEC
 endif

git-compat-util.h

@@ -389,8 +389,13 @@ void git_qsort(void *base, size_t nmemb, size_t size,
 #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