Enforce pre-declaration

[claude-betz]

Currently a block like this doesn't require pre-declaration of func(). It should

int
main()
{
    int *i;
    int *j;

    i = func();
    j = *i;     /* ERROR: unjusitified indirection */

    return 0;
}

int *
func()
{
    int *p;
    p = malloc(1);
    free(p);    /* p dangling */

    return p;   /* return dangling ptr */
}

[claude-betz]

implemented as part of #32