Fixes mantis-2307

Also use FC's Makefile standard variables whenever possible

- a named void parameter is considered an error;
- a void parameter (other than by itself) is considered an error

Since -wp-bool-range is activated by default…

The function Cvalue.V.cast_int_to_int is unsound here, as conversions to the
_Bool type obey a different rule. This case never happens in the C AST, as
conversions (_Bool)x are translated into (_Bool)(x != 0).