include handler

[romainfrancois]

This comes up with the Rcpp impl of attributes. files that work with sourceCpp don't work when making a package because of things not included or defined.

Perhaps we could declare things to include in the generated file

// [[ start_include ]]
using namespace arma ;
// [[ end_include ]]

not really happy about the interface, but that's the idea.

or perhaps something to declare that an include file has to also be included in the generated file:

// [[propagate]]
#include <foo/bar/yada.h>