Hello!
In the libraries you can include “avformat-55”. Now I have downloaded the latest version (avformat-58) from ffmpeg.org. Sadly, it does not allow to embed it. Audacity is unfortunately not future-oriented, so does not believe that plugins evolve.
Would be nice if this would be adapted.
Thanks. 
Updating libraries is an ongoing task. Audacity uses a lot of third party libraries, which are updated periodically. Updating libraries takes developer time, which is limited as there is only a very small team of developers.
If you are an experienced software developer and would like to contribute, see: https://www.audacityteam.org/community/developers/