gettext features missing

The Linux and macOS builds don't link against `libintl` and don't have all `gettext` features present in their `_locale` extension / `locale` Python module.

We could potentially build against our own `gettext` (and the build system has support for this because it was formerly implemented on macOS). However, this has licensing implications as `gettext` is GPL version 3. We'd likely compile an extension module variant for `_locale` that isn't linked with `gettext` so downstream consumers could choose a non-GPL encumbered variant.