add idn for MacOS and iOS

Hello,

I am happy to make a pull request for a little change in Curl_idnconvert_hostname function.

See pull request #5330
https://github.com/curl/curl/pull/5330

I had problem that libidn is not included in MacOS and while I can build it as separate library, the LGPL license doesn't allow me to directly link it into the curl library.

For Linux I wrote myself a little stub to dynamically load libidn when needed/available, so we can use curl with or without it.
(Maybe interesting to include with curl?)

For MacOS since a few versions the system accepts UTF-8 Domain names. So DNS works, but CURL uses wrong hostname for the request.
e.g.

GET / HTTP/1.1
Host: www.müller.de
Accept: */*

With the change proposed, CURL will pick the host name from DNS and then make correct query:

GET / HTTP/1.1
Host: www.xn--mller-kva.de
Accept: */*

I hope this helps and works.
Works for me and I got all changed to make "make checksrc" happy.

Sincerely
Christian

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