configure: detect IPv6 support on Windows

This patch was "nicked" from the MINGW-packages project by Daniel.

https://github.com/Alexpux/MINGW-packages/commit/9253d0bf58a1486e91f7efb5316e7fdb48fa4007
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
2 files changed