The implementation is simplified.
If no implementation of `::atomic` is available, `CGAL_NO_ATOMIC` is
defined to an error message (replaced the `#error`).
- Change the way the macros `CGAL_CAN_USE_CXX11_THREAD_LOCAL` and
`CGAL_CAN_USE_CXX11_MUTEX` are defined, so that other compilers are
also supported.
- Add the macro `CGAL_CAN_USE_CXX11_ATOMIC` and the header
`<CGAL/atomic.h>`.