OSX: Fixup warnings and clock_gettime() bug

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/os/os-mac.h b/os/os-mac.h
index 0f351a7..c8d7dca 100644
--- a/os/os-mac.h
+++ b/os/os-mac.h
@@ -16,10 +16,6 @@
 
 #include "../file.h"
 
-#ifndef CLOCK_MONOTONIC
-#define CLOCK_MONOTONIC 1
-#endif
-
 #ifndef CLOCK_REALTIME
 #define CLOCK_REALTIME 1
 #endif
@@ -177,4 +173,11 @@
 {
 	return mach_thread_self();
 }
+
+/*
+ * For some reason, there's no header definition for fdatasync(), even
+ * if it exists.
+ */
+extern int fdatasync(int fd);
+
 #endif