Dehardcode path to `pkg-config`

Hello!

I'd like to avoid hardcoding the path to `pkg-config`, nowadays is `/usr/bin/pkg-config`. This is a bad practice for the Nix ecosystem, which populates `PATH` so that the executable is accessible without need of a full path. I'm providing a one liner PR for it (#1342), but let me know if a more elaborate solution would be needed.