On head do not build deprecated IPV6-MIB implementations in mibII/ipv6.c,
now superseeded by the INET-XXX-MIB versions. And no need for _WANT_IFADDR
hack now.

This fixes build on head, since cuts away the main kvm(3) abuser from
net-snmp.

Discussion: http://sourceforge.net/p/net-snmp/mailman/message/33539005/
Sponsored by: Netflix
Sponsored by: Nginx, Inc.
Approved by: zi