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

Index: src/libguac/pool.c
--- src/libguac/pool.c.orig
+++ src/libguac/pool.c
@@ -41,7 +41,9 @@ guac_pool* guac_pool_alloc(int size) {
 
     /* Init lock */
     pthread_mutexattr_init(&lock_attributes);
+#ifndef __OpenBSD__
     pthread_mutexattr_setpshared(&lock_attributes, PTHREAD_PROCESS_SHARED);
+#endif
     pthread_mutex_init(&(pool->__lock), &lock_attributes);
 
     return pool;