See also:
https://gitlab.com/vylen/ports/extra/-/blob/main/libotr/libotr-4.1.1-fix-build-with-libgcrypt-1.10.patch
https://github.com/rxrbln/t2sde/blob/eaa768f6543913277a43b93799f668935e2f4368/package/network/libotr/hotfix-socket.patch
https://github.com/NixOS/nixpkgs/blob/ea9e3ef341e11c193537f674f821a99f9f436729/pkgs/by-name/li/libotr/fix-regtest-client.patch

--- libotr-4.1.1/tests/regression/client/client.c.orig
+++ libotr-4.1.1/tests/regression/client/client.c
@@ -26,6 +26,7 @@
 #include <stdlib.h>
 #include <sys/epoll.h>
 #include <sys/types.h>
+#include <sys/socket.h>
 #include <sys/un.h>
 #include <unistd.h>
 
--- libotr-4.1.1/tests/regression/client/Makefile.am.orig
+++ libotr-4.1.1/tests/regression/client/Makefile.am
@@ -11,6 +11,6 @@
 noinst_PROGRAMS = client
 
 client_SOURCES = client.c
-client_LDADD = $(LIBTAP) $(LIBOTR) -lpthread @LIBGCRYPT_LIBS@
+client_LDADD = $(LIBTAP) $(LIBOTR) -lpthread -lsocket @LIBGCRYPT_LIBS@
 
 EXTRA_DIST = otr.key