fix minor bug in handling of 'command not found' type errors in configure script
fix minor bug in handling of 'command not found' type errors in configure script

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3