WIP: add cppgsl explicitely to include_directories
cf LBCORE-1630
This is somewhat needed for !1302 (closed), yet given I don't know why the inclusion of cppgsl has been implemented the way it is (and how it is meant to play together with gaudi_project
) aaand given I don't fully understand why the first line of these two changes is needed, I'd like @clemenci to have a look.