Add FIO_PREFERRED_CLOCK_SOURCE to allow selection of clock source on a per-platform basis.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/gettime.c b/gettime.c
index cf0d664..e8f5ab5 100644
--- a/gettime.c
+++ b/gettime.c
@@ -24,7 +24,7 @@
 int fio_gtod_cpu = -1;
 static pthread_t gtod_thread;
 
-enum fio_cs fio_clock_source = CS_GTOD;
+enum fio_cs fio_clock_source = FIO_PREFERRED_CLOCK_SOURCE;
 
 #ifdef FIO_DEBUG_TIME