Merge: Pkgconfig

Merge: Pkgconfig

Generalize the usage of pkg-config so that this command is executed by the Makefile and not the nitg compiler.

This will improve the portability of programs when the .nit_compile directory is distributed.
And thus, the initial bootstrap with `c_src`.

This is a first step toward the resolution of #864.

Pull-Request: #874
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>