std::codecvt_utf8 is deprecated in C++ 17

[thebearon]

As used eg. here:

libnumbertext/src/Numbertext.cxx

Line 28 in 925e638

wif.imbue(std::locale(std::locale(), new std::codecvt_utf8<wchar_t>));

Workaround to disable the warnings in LO:
https://cgit.freedesktop.org/libreoffice/core/commit/?id=e37e1a7d2007bd6896027b69271774da2568e6f8