Windows issues, msmpi support, symlinking to mpi executable

* windows: don't try rpath in windows
* windows: set ELMERGUI_HOME to $INSTDIR\share\ElmerGUI
* windows: builds with hacked msmpi
* windows: msmpi redist is bundled in nsis installer
* windows: for mpi build ElmerSolver_mpi.exe is copied to ElmerSolver.exe
* unix: for mpi build ElmerSolver is symlinked to ElmerSolver_mpi
* MPI_FOUND workaround for windows. On windows MPI is found but MPI_FOUND is not set