<codecvt> is deprecated – optionally allow using ICU?

[TinoDidriksen]

Header and the related functions are deprecated as of C+17 - https://en.cppreference.com/w/cpp/header/codecvt

It is used in at least two places in the code ( https://github.com/divvun/libdivvun/search?q=codecvt ).

[unhammer]

Deprecated without a replacement, even :-(

It's used in getCasing and toupper / tolower in order to convert utf-8 strings to wstrings so we can call isupper/islower/towupper on non-latin1 stuff. As suggest.hpp says,

// #ifndef icu TODO

[unhammer]

Since making a native locale fails in LO on mac (even though it shouldn't?) we may have a functionality-reason to start using ICU too.