Building modules packaged with Janet does not work when Janet is not installed in system directory

[janet-lang]

Building modules packaged with Janet does not work when Janet is not installed in system directory

[bakpakin]

I think by default, the correct and easiest thing to do would be to use what include path was set while building Janet (it is assumed here that Janet is installed somewhere). Janet already has module/*syspath*, which is where it looks for global modules. We could also add something like module/*headerpath*, which is where it could look for janet.h.

[bakpakin]

The latest commits should address this problem. We simply put the header in the library path, and the cook tool adds the correct include flag as needed.