Add IDN support to our static libcurl build

[ChristianBeer]

International Domain Names (Umlaut-Domains among others) may be a thing in the future. libcurl already supports libidn2 which translates the domain name into a string that can be resolved. Since we build a static version of libcurl we also need to build a static version of libidn2 too.

Currently this would need to be implemented for Windows and Mac since we don't really have a static Linux build.