Identification of ffmpeg nightly builds

[midas02]

Some users will/might be running nightly builds of ffmpeg, identified with a version number nx.y.z. This change will help the code to identify those builds correctly and make the detection more robust.

[dinkypumpkin]

Thanks for the PR. Unfortunately, I cannot merge it. The whole of point of that bit of code is to identify only release builds so that appropriate warnings can be given later based on media stream format. There is no guaranteed way to know when a nightly or other custom build was made, or what capabilities it has, or how it relates to a given release. I merely assume that people using nightlies or other custom builds know what they are doing and show a message as a reminder. On a more practical level, your regex would not identify many or most nightly builds.