gettext features missing

[indygreg]

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.