https://bugs.gentoo.org/904740
https://github.com/ppp-project/ppp/pull/319

From 44a70d6f9eb6d1843868da1d45de382134a03630 Mon Sep 17 00:00:00 2001
From: str8fast <52187302+str8fast@users.noreply.github.com>
Date: Thu, 28 Oct 2021 14:55:56 +0000
Subject: [PATCH] pppol2tp.c add '#ifdef INET6' for optional ipv6

Without it, l2tp daemon can't launch ppp, cuz undefined symbol ipv6_up_notifier.
---
 pppd/plugins/pppol2tp/pppol2tp.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/pppd/plugins/pppol2tp/pppol2tp.c b/pppd/plugins/pppol2tp/pppol2tp.c
index ed2d7c79..c9902afe 100644
--- a/pppd/plugins/pppol2tp/pppol2tp.c
+++ b/pppd/plugins/pppol2tp/pppol2tp.c
@@ -509,8 +509,10 @@ void plugin_init(void)
 	 */
 	add_notifier(&ip_up_notifier, pppol2tp_ip_up, NULL);
 	add_notifier(&ip_down_notifier, pppol2tp_ip_down, NULL);
+#ifdef INET6
 	add_notifier(&ipv6_up_notifier, pppol2tp_ip_up, NULL);
 	add_notifier(&ipv6_down_notifier, pppol2tp_ip_down, NULL);
+#endif
 }
 
 struct channel pppol2tp_channel = {