The ffmpeg version from fedora-handbrake repo is the one that needs that ilbc library.
This is the version I have installed:
Name : ilbc Arkkitehtuu : x86_64 Epoch : 0 Versio : 1.1.1 Release : 9.fc24 Koko : 98 k Repo : @System From repo : fedora Summary : Internet Low Bitrate Codec License : BSD Description : iLBC (internet Low Bitrate Codec) is a FREE speech codec suitable : for robust voice communication over IP. The codec is designed for : narrow band speech and results in a payload bit rate of 13.33 : kbit/s with an encoding frame length of 30 ms and 15.20 kbps with : an encoding length of 20 ms. The iLBC codec enables graceful : speech quality degradation in the case of lost frames, which : occurs in connection with lost or delayed IP packets.
Which may be the last version in 1.x series. You probably have a version from the 2.x series. They are probably not binary compatible, thus the last digit in the library file name has been changed to a 2. But if you can't compile or otherwise get an older version you could instead delete 'libavcodec' and other ffmpeg libraries from the thrive folder and install ffmpeg 3.3 (that's the very latest version and it is required, most distributions probably won't have that)