Check for compiling for IPv6; don't check whether we can create an IPv6 socket.

Check for compiling for IPv6; don't check whether we can create an IPv6 socket.

Don't check whether we can create a TCP-over-IPv6 socket; that appears
to fail in some cases where tcpdump can be built with IPv6 support.
Instead, just check whether AF_INET6 and struct in6_addr are defined.