configure: assume IPv6 works when cross-compiled

The configure test uses AC_TRY_RUN to figure out if an ipv6 socket
works, and testing like that doesn't work for cross-compiles. These days
IPv6 support is widespread so a blind guess is probably more likely to
be 'yes' than 'no' now.

Further: anyone who cross-compiles can use configure's --disable-ipv6 to
explicitly disable IPv6 and that also works for cross-compiles.

Made happen after discussions in issue #594