When `CGAL_HEADER_ONLY` and `WITH_{examples|tests|demos}`, then only
the first call to `find_package(CGAL)` does the job. The subsequent
calls return very fast, by caching the results in global properties.
- Now even CGAL_Core has one.
- The `${cgal_lib}_FOUND` are now set in those modules, after having chec
- For self-consistency, `Use_CGAL_Qt5_headers` is included by
`CGAL_SetupCGAL_Qt5Dependencies`.
That is the step 2 of the header-only effort.
Still to do:
- make it work with `make install`
- ensure that `CGAL_Core` is correctly handled with `CGAL_DISABLE_GMP`