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`
the message is a bit confusing. if the MPI lib was a binary package, there was no compiler `used to install` in the first place. the toolchain needs a compiler (and `mpicc` wrappers also), but the installation of the MPI libs is not related to that