Fix for Mac build.

Fake CLOCK_REALTIME.

Change-Id: I82550dec0d67a4d434ff035f8a922c8c5d6e56ca
diff --git a/src/mutex.cc b/src/mutex.cc
index cbf115e..ec0752e 100644
--- a/src/mutex.cc
+++ b/src/mutex.cc
@@ -29,6 +29,8 @@
 
 #if defined(__APPLE__)
 #include "AvailabilityMacros.h" // For MAC_OS_X_VERSION_MAX_ALLOWED
+// No clocks to specify on OS/X, fake value to pass to routines that require a clock.
+#define CLOCK_REALTIME 0xEBADFOOD
 #endif
 
 #define CHECK_MUTEX_CALL(call, args) CHECK_PTHREAD_CALL(call, args, name_)