BUILD: tcp: use IPPROTO_IPV6 instead of SOL_IPV6 on FreeBSD/MacOS

Lukas reported in issue #1203 that the previous fix for silent-drop in
commit ab79ee8b1 ("BUG/MINOR: tcp: fix silent-drop workaround for IPv6")
breaks the build on FreeBSD/MacOS due to SOL_IPV6 not being defined. On
these platforms, IPPROTO_IPV6 must be used instead, so this should fix
it.

This needs to be backported to whatever version the fix above is backported
to.

(cherry picked from commit da2319578555e56eafab0141505ce943f954d859)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/src/tcp_act.c b/src/tcp_act.c
index 26ce65f..6a9ebf7 100644
--- a/src/tcp_act.c
+++ b/src/tcp_act.c
@@ -211,8 +211,13 @@
 		setsockopt(conn->handle.fd, SOL_IP, IP_TTL, &one, sizeof(one));
 #endif
 #ifdef IPV6_UNICAST_HOPS
+#if defined(SOL_IPV6)
 	if (conn->src && conn->src->ss_family == AF_INET6)
 		setsockopt(conn->handle.fd, SOL_IPV6, IPV6_UNICAST_HOPS, &one, sizeof(one));
+#elif defined(IPPROTO_IPV6)
+	if (conn->src && conn->src->ss_family == AF_INET6)
+		setsockopt(conn->handle.fd, IPPROTO_IPV6, IPV6_UNICAST_HOPS, &one, sizeof(one));
+#endif
 #endif
  out:
 	/* kill the stream if any */