compileAttributes does not support the [[export]] attributes

@kevinushey when I try to use `[[export]]` instead of `[[Rcpp::export]]` I get: 

```
Erreur dans attributes$attributes[sapply(attributes$attributes, function(x) { :
  type 'list' d'indice incorrect
Calls: compileAttributes ... compileDefaultAttributes -> unlist -> lapply -> FUN
```
