Generalize IPV6_V6ONLY usage

Some platforms other than Linux may implement this socket option.

config.sh

@@ -30,12 +30,6 @@ do
 	esac
 done
 
-case "$(uname)" in
-	Linux)
-		CFLAGS="$CFLAGS -DLINUX"
-		;;
-esac
-
 subdir() {
 	eval ". $srcdir/$1/configure"
 }

src/server.c

@@ -50,7 +50,7 @@ server_init(struct gmnisrv_server *server, struct gmnisrv_config *conf)
 			in.sin6_port = htons(b->port);
 			addr = (struct sockaddr *)∈
 			addrsz = sizeof(in);
-#ifdef LINUX
+#ifdef IPV6_V6ONLY
 			static int t = 1;
 			setsockopt(sockfd, IPPROTO_IPV6,
 				IPV6_V6ONLY, &t, sizeof(t));