It's (almost, but is for all practical purposes) documented as using g_iconv(), which is documented as using iconv() or libiconv as fallback.
Ok, the [code-ocumentation](https://github.com/GNOME/glib/blob/main/glib/gconvert.c) shows its iconv all the way, not a #ifdef windows in sight :-)
just use g_iconv_open(A, B) and it'll error out with EINVAL if the conversion from B to A is not supported.
So we can use that in `init_encodings()` to remove unsupported ones from the list.
It won't add any not supported by whoever made the list in `encodings.c`, but hopefully in this time of Unicode that won't be many.