fix hierarchical mns w.r.t. MPI installed with a dummy toolchain

enhanced version of #983 by @kcgthb (which should have been opened targeting `develop`
This part still needs to be figured out...

I can't think of a way to determine the right path here, since we have no information at this point with which compiler(s) the MPI library will be used with a this point.

So, we need some kind of generic path in that case?

@kcgthb: thoughts?