Custom file paths exclusion for cppcheck

This commit defines and manage a CMakeList variable that enables a custom exclusion of source files when running ccpcheck by means of "make cppcheck" or "make test".

It has been successfully tested with the HBP project named neurorobotics/ESVRender for which we wanted to exclude large source files imported from external software (namely some flawed  OpenSceneGraph's shadowmap files) that had undergone very small modifications.