Refine libvlc header versioning
So far the libvlc version is following exactly the VLC version. But in 4.0 there has been a lot evolutions/additions/changes and the builds always say it's libvlc 4.0.0. It's makes maintaining code using libvlc from a 4.0 branch tricky to maintain. The same wrapper can only be used with a libvlc from a given time. There's no way to detect changes (latest is !2325 (merged)) from looking at headers. You can never go back and compile your current code (to find regressions for examples).
Either we update the "revision" versions in VLC 4.0.0 branches which will be reflected in the libvlc_version.h
as well. Or we start maintaining a libvlc API version separately which would be updated when changes are done to it (similar to how the libavcodec API is maintained).
I favor the latter approach. If we release a VLC 4.0.1 this doesn't mean the libvlc has changed. On the other hand there may be many libvlc changes between two releases and it would be good to be able to maintain code that works with interim versions.