Support overriding in Cargo config

[kryptan]

I have built ffmpeg myself and want to use it with ffmpeg-sys. I'm overriding build script as described here. The issue is that build script in addition to actually building the library also detects its features. If I override the build script I will have to manually specify all these features which is error prone.

To allow overriding I think that ffmpeg-sys must be split into two crates.

1. ffmpeg-sys-builld will only contain a build script to build ffmpeg (or find an existing installation) and will declare links value in it's Cargo.toml.
2. ffmpeg-sys will depend on ffmpeg-sys-builld and contain build script which detects features.

This way it will be possible to use overriding mechanism as it will only override the build of ffmpeg (in ffmpeg-sys-builld/build.rs) but feature detection (in ffmpeg-sys/build.rs) will work as usual.

[bacek]

Second it. I would like to link with system ffmpeg packages instead of building ffmpeg from scratch.