fix without IPv6

see https://github.com/luaposix/luaposix/pull/117

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>

Index: b/ext/posix/posix.c
===================================================================
--- a/ext/posix/posix.c
+++ b/ext/posix/posix.c
@@ -3637,6 +3637,7 @@
 					break;
 			}
 			break;
+#if defined(IPV6_JOIN_GROUP) && defined(IPV6_LEAVE_GROUP)
 		case IPPROTO_IPV6:
 			switch(optname) {
 				case IPV6_JOIN_GROUP:
@@ -3650,6 +3651,7 @@
 					break;
 			}
 			break;
+#endif
 		case IPPROTO_TCP:
 			switch(optname) {
 				default:
@@ -4697,13 +4699,27 @@
 	MENTRY( AI_V4MAPPED	);
 	MENTRY( AI_ALL		);
 	MENTRY( AI_ADDRCONFIG	);
+#endif
 
+#ifdef IPV6_JOIN_GROUP
 	MENTRY( IPV6_JOIN_GROUP		);
+#endif
+#ifdef IPV6_LEAVE_GROUP
 	MENTRY( IPV6_LEAVE_GROUP	);
+#endif
+#ifdef IPV6_MULTICAST_HOPS
 	MENTRY( IPV6_MULTICAST_HOPS	);
+#endif
+#ifdef IPV6_MULTICAST_IF
 	MENTRY( IPV6_MULTICAST_IF	);
+#endif
+#ifdef IPV6_MULTICAST_LOOP
 	MENTRY( IPV6_MULTICAST_LOOP	);
+#endif
+#ifdef IPV6_UNICAST_HOPS
 	MENTRY( IPV6_UNICAST_HOPS	);
+#endif
+#ifdef IPV6_V6ONLY
 	MENTRY( IPV6_V6ONLY		);
 #endif
 #undef MENTRY