Since upstream plans making a new release this weekend I wanted to speed up fixing all the FTBFS-s by asking for help.
The fix have to be different one - "sockaddr6_in -> sockaddr_in6":
--- lib/socket.c~
+++ lib/socket.c
@@ -506,7 +506,7 @@
                                        ((struct sockaddr_in6 *)&ss)->sin6_port = port;
                                        ((struct sockaddr_in6 *)&ss)->sin6_family      = AF_INET6;
 #ifdef HAVE_SOCKADDR_LEN
-                                       ((struct sockaddr_in6 *)&ss)->sin6_len = sizeof(struct sockaddr6_in);
+                                       ((struct sockaddr_in6 *)&ss)->sin6_len = sizeof(struct sockaddr_in6);
 #endif
                                        break;
Cheers
	Petr