[FFmpeg-devel] configure: Fix decklink license dependency

[Elliott Balsley]

> It is not Blackmagic who does not want you to distribute FFmpeg
> with their (proprietary) library, it is us who do not allow distribution
> of FFmpeg binaries that link against Blackmagic libraries.


When you say proprietary library, do you mean the Desktop Video driver?  I believe this qualifies as a system library, so this situation is quite similar to NVENC, which uses open source headers and requires a closed source Nvidia driver to function.
https://ffmpeg.org/pipermail/ffmpeg-devel/2016-April/193467.html <https://ffmpeg.org/pipermail/ffmpeg-devel/2016-April/193467.html>