Using pkg-config or similar for link dependencies

[skadisch]

Currently the ffmpeg libraries are linked using e.g. "-lavcodec". This can lead to problems, depending on the compile options of ffmpeg. Using "<!(pkg-config --libs libavcodec)" would solve this issue (that we face in our setup), at least on Linux platforms.

Would it be legit to use pkg-config on non-Windows systems?

I would be happy to provide a PR.