add -DATLAS to compiler flags for ATLAS
for ATLAS hive builds, we need to add the -DATLAS C++ compile option. this patch adds this to build option GAUDI_ATLAS
this should probably also be added to the master branch
for ATLAS hive builds, we need to add the -DATLAS C++ compile option. this patch adds this to build option GAUDI_ATLAS
this should probably also be added to the master branch