Support @@cfg in function branches

[leostera]

Currently, this is unsupported:

let foo a = function
  | 0 -> "0"
  | 1 -> "1"
  [@@cfg (disable=true)]

And the expected behavior is that we can remove that function branch with the annotation.

[KFoxder]

This will get addressed in https://github.com/ocaml-sys/config.ml/pull/13. @leostera you can assign this to me.

[KFoxder]

@leostera I think this too can be closed now