cmake: explain WITH_DLPOLY better
cmake: explain WITH_DLPOLY better

Pallavi-Banerjee21/votca.csg,vaidyanathanms/votca.csg,Pallavi-Banerjee21/votca.csg,Pallavi-Banerjee21/votca.csg,vaidyanathanms/votca.csg,vaidyanathanms/votca.csg,Pallavi-Banerjee21/votca.csg,vaidyanathanms/votca.csg,vaidyanathanms/votca.csg,Pallavi-Banerjee21/votca.csg,Pallavi-Banerjee21/votca.csg