Not supported on OpenBSD, but guacd seems to run OK without this anyway.

Index: src/libguac/client.c
--- src/libguac/client.c.orig
+++ src/libguac/client.c
@@ -172,7 +172,9 @@ guac_client* guac_client_alloc() {
 
     /* Init locks */
     pthread_rwlockattr_init(&lock_attributes);
+#ifndef __OpenBSD__
     pthread_rwlockattr_setpshared(&lock_attributes, PTHREAD_PROCESS_SHARED);
+#endif
 
     pthread_rwlock_init(&(client->__users_lock), &lock_attributes);