configury: fix opal_config_pthreads when -lrt is set

the test program was successful without -pthread if -lrt is used.
-pthread *is* required in order to correctly handle
the pthread_atfork function.

config/opal_config_pthreads.m4

@@ -58,6 +58,7 @@ int main(int argc, char* argv[])
     pthread_attr_t attr;
 
     me = pthread_self(); 
+    pthread_atfork(NULL, NULL, NULL);
     pthread_attr_init(&attr); 
     pthread_cleanup_push(cleanup_routine, 0);
     pthread_create(&newthread, &attr, thread_main, 0);