Fix: runtime patch for when system has clock_gettime but does not support CLOCK_MONOTONIC.

src/clock.cpp

@@ -70,7 +70,16 @@ uint64_t zmq::clock_t::now_us ()
     //  Use POSIX clock_gettime function to get precise monotonic time.
     struct timespec tv;
     int rc = clock_gettime (CLOCK_MONOTONIC, &tv);
-    errno_assert (rc == 0);
+		// Fix case where system has clock_gettime but CLOCK_MONOTONIC is not supported.
+		// This should be a configuration check, but I looked into it and writing an 
+		// AC_FUNC_CLOCK_MONOTONIC seems beyond my powers.
+		if( rc != 0) {
+			//  Use POSIX gettimeofday function to get precise time.
+			struct timeval tv;
+			int rc = gettimeofday (&tv, NULL);
+			errno_assert (rc == 0);
+			return (tv.tv_sec * (uint64_t) 1000000 + tv.tv_usec);
+		}
     return (tv.tv_sec * (uint64_t) 1000000 + tv.tv_nsec / 1000);
 
 #else