Correct document generation in docs.rs

[Donough Liu]

It is a really interesting problem that the docs.rs successfully generated documentation for version 0.1.0 but failed to generate documentation for version 0.2.0.

That's because the 0.1.0 version tried to probe FFmpeg in the system if no user-compiled FFmpeg is used. But 0.2.0 just uses the provided FFmpeg or compiled FFmpeg by itself. The reason why 0.1.0 successfully generates bindings is that the docs.rs' docker image contains libav* libraries, you can check rust-lang/docs.rs#851 for the discussion.

So now for 0.2.0 to successfully generate useful documentation, there are several hacks need to be done on the build script. And the most important thing is docs.rs detection in build script. But currently it's not possible. So I filed a PR to fix it in docs.rs, so currently we are blocking on it's merge.

[Donough Liu]

With rust-lang/docs.rs#852 got merged, this is not blocked any more.