[plt-scheme] C enum syntax

I've been using the C FFI and handling enumerated constants like so, which seems
to work fine:

   (define some-const ((c-lambda () int "___result = SOME_CONST;")))

But I would rather do something like:

   (define some-const (c-enum "SOME_CONST"))

So I tried:

   (define-syntax (c-enum stx)
     (syntax-case stx ()
       ((_ c)
        (quasisyntax
         ((c-lambda () int 
            (unsyntax (format "___result = ~a;" (syntax c)))))))))

Which is apparently not correct.  Could someone shed some light on what's wrong
with the `c-enum' syntax above?  Also, is there a better approach to enums in
scheme?

Thanks,
David