Check `*-config --libs` commands too.

Since I want to use it in rust-mecab.
You're right it was confusing.

[This](https://gist.github.com/take-cheeze/6d6c37169516513459c2) is a error message that separates `e_cfg`, `e_pkg` message and replaces "pkg-config" with stringified command.
Is it recognizable enough?