changing functions to a separate header, not included by default
(<CGAL/assertions_behaviour.h>). The motivation is to hide the enum
values with risky names (ABORT, EXIT, CONTINUE) to a header file which
is most probably not used by any user (or very few).
(breaks backward compatibility for an expected very few, for the sake
of erasing random surprises for "many" ?)
Sylvain Pion