Add support for libidn2

[DomT4]

libidn2 is stable now, and slowly being adopted more widely. Here's a patch to add support for it as desired.

Some projects are simply dropping libidn 1.x support like curl and wget, but a minimally-invasive change by logic would seem to have a better chance of not being rejected. Tested with ./configure --with-libidn2 --without-libidn and it built fine.

Please feel free to sanity check this, I do not claim to do C well, and stupidity is not above me 😄.

[DomT4]

The Travis build shows no log at all, so I'm a bit confused as to why this failed. It builds on macOS 10.12 locally, FWIW.

[AdamWill]

FWIW, Fedora would like to have this too. https://bugzilla.redhat.com/show_bug.cgi?id=1449130

[andersk]

The Travis build log shows up fine for me.

checking for library containing stringprep_check_version... -lidn
checking for library containing idn2_check_version... no
configure: error: --with-libidn2 was given but libidn2 was not found.

[DomT4]

I haven’t looked at this since I filed the initial PR here so I can’t vouch for how compliant it is with the latest releases. I’ll try and glance over the code again over in the next couple days.

[DomT4]

Still seems to build & link fine locally against libidn2 2.0.5 FWIW, but I don't have high hopes of it getting human review given how long this has sat now.

[nmav]

Note that travis uses a very old ubuntu with a very old libidn2 which doesn't contain the function used in this patch. Also in the linking part I see that libidn is linked in addition to libidn2 which most likely is the failure reason.