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`
I think that would be a good solution. 

But here is something I don't understand very well: since the intel toolchain now depends on GCC, if you decide to install impi with the intel toolchain, it means it will depend on both GCC and icc?