`ffi` no longer installs

Hey there - I believe there's been a regression in this feedstock. Recently I started getting CircleCI build failures when using `conda install rb-github-pages`:

https://circleci.com/gh/jupyter/jupyter-book/2229

It seems like there's a problem with building native extensions (I'm assuming there's some kind of library linking problem). Has anybody else (maybe @sodre) run into this?

Start by opening the linked CircleCI build for jupyter/jupyter-book/2229 and reproducing the conda install rb-github-pages failure. Investigate the reported ffi native-extension or library-linking failure in the feedstock, then verify that the package installs successfully in the same build environment.

Written by the indexing model from the issue text.