ConvertUTF.{cpp,h} are non-free

[cstrahan]

The ConvertUTF.{cpp,h} are non-free (and buggy/unmaintained), as discussed on the Debian mailing list:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=823100

This poses problems for packaging/redistribution. Would it be possible to, perhaps, support linking with libicu for this purpose?

We're currently using linenoise-ng in Nix, and this issue is blocking an effort to package Nix for Debian. We could always fork linenoise-ng to use libicu, but considering that all software using linenoise-ng right now are built with non-free code, I figure it makes most sense to have this resolved here.

Thanks!