bool to_int() will return 0 or 1, useful if one wants to set
a define to 0 or 1 based on a boolean result instead of having
it just defined or undefined.
bool to_string() will return 'true' or 'false' by default same
as when using it to format a string, but with the additional
possibility to specify two extra string arguments to be returned
as true/false values, e.g. to_string('yes', 'no'). This can be
useful when outputting messages to be shown to the user.