- move `CGAL_WARNING` in `<CGAL/config.h>`,
- create the macro `CGAL_pragma_warning`,
- use `CGAL_pragma_warning` to simplify `<CGAL/license/lgpl.h>` and
`<CGAL/license/gpl.h.in>`.
- add messages in `include/CGAL/license/generate_files.cmake`
- add test files, that do nothing but can be modified to check errors
and warning.