only define clock macros on darwin if not already defined

clang from Xcode 8 has these already defined, causing duplicate macro warnings

src/clock.hpp

@@ -38,9 +38,13 @@
 #include <time.h>
 #include <sys/time.h>
 int clock_gettime (int clock_id, timespec *ts);
+#ifndef CLOCK_REALTIME
 #define CLOCK_REALTIME 0
+#endif
+#ifndef HAVE_CLOCK_GETTIME
 #define HAVE_CLOCK_GETTIME
 #endif
+#endif
 
 namespace zmq
 {