ax_gromacs.m4: minor fix for AIX
ax_gromacs.m4: minor fix for AIX

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