Use libiconv in gettext

If libiconv is not linked to gettext, I get the following error during
the compilation of gcc-10.5 (in a system with minimal requirements):

```
INFO msgfmt -o de.mo ../../.././libstdc++-v3/po/de.po
INFO msgfmt: Cannot convert from "ISO-8859-1" to "UTF-8". msgfmt relies on iconv(). This version was built without iconv().
```

See merge request !1732