Probe for <stdbool.h>, and if found use it in handy.h

Probe for <stdbool.h>, and if found use it in handy.h

This means that the core uses the compiler's bool type if one exists.
This avoids potential problems of clashes between perl's own implementation
of bool and the compiler's bool type, which otherwise occur when one
attempts to include headers which in turn include <stdbool.h>.

Signed-off-by: H.Merijn Brand <h.m.brand@xs4all.nl>