gettime: use pthread_{set,get}specific() for TLS

__thread doesn't work on OSX at least (thanks...), so resort to
using the pthread functions to maintaining a per-thread clock
check.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/gettime.h b/gettime.h
index 8ca3791..f0ad20c 100644
--- a/gettime.h
+++ b/gettime.h
@@ -16,6 +16,7 @@
 extern void fio_clock_init(void);
 extern int fio_start_gtod_thread(void);
 extern int fio_monotonic_clocktest(void);
+extern void fio_local_clock_init(int);
 
 extern struct timeval *fio_tv;