Libidn2 is a GNU internationalized domain name (IDN) processing library
implementing the IDNA2008 specification (libidn implements IDNA2003).

This library is ostensibly used by the most recent release of curl,
although the curl maintainer has suggested distributors disable IDN
support altogether for now:

https://curl.haxx.se/mail/lib-2016-11/0033.html

In any case, here is a package definition for the latest libidn2
release.