ax_gromacs.m4: better error message
ax_gromacs.m4: better error message

votca/csg,junghans/csg,MrTheodor/csg,MrTheodor/csg,MrTheodor/csg,junghans/csg,votca/csg,votca/csg,votca/csg,junghans/csg,junghans/csg,junghans/csg,junghans/csg,MrTheodor/csg,MrTheodor/csg