Adapt js_of_ocaml ppx example.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13566 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Adapt ifdef example.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13567 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02