From: ac697f5bac (r31310663)
It's not easy to reliably parse/guess the extension for manually
uploaded archives using GitHub releases. Even if the naming convention
matches GitHub's automated rules, there are two problems:
- mxe version may be on the ["next" page](https://github.com/fribidi/fribidi/releases?after=0.19.4)
- using API may hit rate limiting if it's a live check at build time
This adds an "archive extension" variable to `GH_CONF` for the simple
cases where all other naming follows GitHub conventions.
Mark Brand
Tony Theodore