Dear Justin Maggard,
TBH I don't have enough knowledge about the libav / ffmpeg split to be certain that this is a "bug" in ReadyMedia, maybe you only want ffmpeg support? (Does minidlna actually work with ffmpeg? No idea!)
Anyway, I was getting my hopes up that minidlna could finally re-enter Debian testing after an extended period without maintenance (in Debian, stuck at 1.0.24) and subsequently being removed from Debian testing. It was upgraded to 1.1.2 very recently and I was optimistic to see it back in Debian testing.
Unfortunately, Debian is currently upgrading libav to version 10. It introduces a few API breaking changes: https://wiki.libav.org/Migration/10
So now minidlna needs to be adjusted to work with libav10. I posted a patch to the debian bug tracker, hoping to speed up minidlna inclusion in Debian testing. I'm not sure if the patch is okay for your upstream git repository since I don't know if it's backwards compatible. Apparently the AV_CODEC_ID_* macros have existed for some time, and possibly the avg_frame_rate as well, so maybe it works for older libav versions too.
Also note that I tried to follow the migration guide, but the patch is only compile-tested. It might still break some functionality.
In case you are interested: