how to make-variable-buffer-local using only defcustom?

[Drew Adams]

Is there a way, using only defcustom, to `make-variable-buffer-local'?

That is, instead of doing a defcustom plus a make-variable-buffer-local, is
there a keyword (:local perhaps?) or some other way to indicate to defcustom
that the variable is to always be buffer-local? For example, instead of
this:

(defcustom foo t "*If nil, then..."
  :type 'boolean :group 'foobar)
(make-variable-buffer-local 'setnu+-fit-frame)

can you do something like this:

(defcustom foo t "*If nil, then..."
  :type 'boolean :local t :group 'foobar)