'sun' is predefined macro. See `cpp -dM < /dev/null`.

--- libotr-4.1.1/tests/regression/client/client.c.orig
+++ libotr-4.1.1/tests/regression/client/client.c
@@ -719,10 +719,10 @@
 					goto end;
 				} else if (event & EPOLLIN) {
 					socklen_t len;
-					struct sockaddr_un sun;
+					struct sockaddr_un address;
 
 					/* Connection from Bob, accept it so we can handle it. */
-					sock_from_bob = accept(fd, (struct sockaddr *) &sun,
+					sock_from_bob = accept(fd, (struct sockaddr *) &address,
 							&len);
 					ret = add_sock_to_pollset(epfd, sock_from_bob,
 							EPOLLIN | EPOLLERR | EPOLLHUP);
@@ -841,10 +841,10 @@
 					goto end;
 				} else if (event & (EPOLLIN | EPOLLHUP)) {
 					socklen_t len;
-					struct sockaddr_un sun;
+					struct sockaddr_un address;
 
 					/* Connection from Alice, accept it so we can handle it. */
-					sock_from_alice = accept(fd, (struct sockaddr *) &sun,
+					sock_from_alice = accept(fd, (struct sockaddr *) &address,
 							&len);
 					ret = add_sock_to_pollset(epfd, sock_from_alice,
 							EPOLLIN | EPOLLERR | EPOLLHUP);
@@ -971,7 +971,7 @@
 }
 
 static int create_unix_socket(const char *pathname,
-		struct sockaddr_un *sun)
+		struct sockaddr_un *address)
 {
 	int sock, ret;
 
@@ -982,12 +982,12 @@
 		goto error;
 	}
 
-	memset(sun, 0, sizeof(struct sockaddr_un));
-	sun->sun_family = AF_UNIX;
-	strncpy(sun->sun_path, pathname, sizeof(sun->sun_path));
-	sun->sun_path[sizeof(sun->sun_path) - 1] = '\0';
+	memset(address, 0, sizeof(struct sockaddr_un));
+	address->sun_family = AF_UNIX;
+	strncpy(address->sun_path, pathname, sizeof(address->sun_path));
+	address->sun_path[sizeof(address->sun_path) - 1] = '\0';
 
-	ret = bind(sock, (struct sockaddr *) sun, sizeof(struct sockaddr_un));
+	ret = bind(sock, (struct sockaddr *) address, sizeof(struct sockaddr_un));
 	if (ret < 0) {
 		perror("bind unix sock");
 		goto error;