Use pkg-config instead of manual -lvlc

[xaionaro]

Summary

pkg-config is usually more aware of how exactly something should be linked, which may vary for different systems/environments.

To make the solution more universal using pkg-config instead of manual flag -lvlc.

Here is an example of another project which already uses this approach: https://github.com/asticode/go-astiav

More specifically, see: https://github.com/asticode/go-astiav/blob/cd2a16de95f911b69a9187708abe58d2c89b4390/astiav.go

Initial motivation

I needed this change because I'm trying to link all the dependencies in one of my projects statically. I was able to hack around this for the go-astiav via manipulating pkg-config, but for this project I had to add this change.