configure: enable IPv6 support without `getaddrinfo`

configure: enable IPv6 support without `getaddrinfo`

This makes it possible to recognize and connect to literal IPv6
addresses when `getaddrinfo` is not available, which is already the
case for the CMake build. This affects e.g. classic MinGW because it
still targets Windows 2000 by default, where `getaddrinfo` is not
available, but general IPv6 support is.

Instead of checking for `getaddrinfo`, check for `sockaddr_in6` as the
CMake build does.

Closes https://github.com/curl/curl/pull/4662