diff -ruN freeciv.orig/Makefile freeciv/Makefile
--- freeciv.orig/Makefile	2010-06-29 21:36:43.000000000 +0200
+++ freeciv/Makefile	2010-06-30 01:48:29.219952813 +0200
@@ -70,6 +70,12 @@
 PLIST_SUB+=	NLS=""
 .endif
 
+.ifdef WITHOUT_IPV6
+CONFIGURE_ARGS+=--enable-ipv6=no
+.else
+CONFIGURE_ARGS+=--enable-ipv6=yes
+.endif
+
 post-patch:
 	@${REINPLACE_CMD} -e '/test.*==/s|==|=|;\
 		/^LIBS/s|mixer|mixer $$SDL_LIBS|g' ${WRKSRC}/configure