(verify): Don't use the __LINE__ trick, as it doesn't work in general.
Instead, declare a function that returns a pointer to an array,
and use verify_type__ to declare the size of the array.
Problem and germ of a solution reported by Bruno Haible.
(verify_type__) [verify_error_if_non_const__]: New member/test,
to help detect when verify or verify_expr is mistakenly passed
a non-constant argument within a function.
depend on whether verify_decl is defined.
(verify): Renamed from verify_decl. All uses changed.
Use an extern function decl, as it can't possibly collide with other
decls.
(verify_expr): Renamed from verify. All uses changed.
(verify_type__): New private macro.
(verify, verify_expr): Use it.
