z/OS: Skip pthread_{get,set}schedparam

Updated:
    src/thread.cpp: On older z/OS UNIX System Services,
        pthread_{get,set}schedparam is not present (searching the
        Internet suggests it may be present in later version than
        the one being used for z/OS UNIX System Services porting).

        Make zmq::thread_t::setSchedulingParameters() a no-op on
        z/OS UNIX System Services.

    NOTE: pthread_{get,set}schedparam appear to have been introduced
    by POSIX.1-2001 or IEEE 1003.1-2004 so may not be universally
    available, and thus more platforms may need this "no-op" treatment.

src/thread.cpp

@@ -104,6 +104,7 @@ void zmq::thread_t::stop ()
 
 void zmq::thread_t::setSchedulingParameters(int priority_, int schedulingPolicy_)
 {
+#if !defined ZMQ_HAVE_ZOS
     int policy = 0;
     struct sched_param param;
 
@@ -122,6 +123,7 @@ void zmq::thread_t::setSchedulingParameters(int priority_, int schedulingPolicy_
 
     rc = pthread_setschedparam(descriptor, policy, &param);
     posix_assert (rc);
+#endif
 }
 
 #endif