[FFmpeg-devel] configure: Fix decklink license dependency

[Elliott Balsley]

I see this old thread about Decklink licensing, and I disagree with the nonfree requirement.
I can’t figure out how to reply to the old thread, but here it is for reference: https://patchwork.ffmpeg.org/patch/3230/ <https://patchwork.ffmpeg.org/patch/3230/>
Downloading the SDK from Blackmagic uses a EULA which prevents distribution, but each individual header file contains a more permissive Boost license.  So apparently, the only part of the SDK covered by the restrictive EULA is the PDF documentation.  This means the headers could even be included in the ffmpeg GitHub if you desire.

I emailed Blackmagic to clarify this, and here is their response:
There is no fee to use the DeckLink SDK. Access to the DeckLink SDK is governed by the EULA and is only available from the Blackmagic Design website. Applications that are built with the SDK are then licensed via the more permissive license contained within the SDK.

It is Blackmagic Design policy that there should only be one publicly available source for the SDK (i.e. the Blackmagic Design website) and in order to download the SDK the EULA must be agreed to. 

In addition, distribution of the SDK is prohibited as per the EULA ("The Licensee must not lease, rent, distribute or sub-license the Software, or use the Software in a time-sharing arrangement, or in any other manner that is not the Permitted Use.”)