Possibly prefix message from R/RcppLdPath.R with Rcpp::: #1456

Brian Ripley just added checks to r-devel ¹ to look for the message we emit from R/RcppLdPath.R for the long-deprecated use of things like LdFlags() (that we reactivated in PR #1413 addressing issue #1412. He notes (in email) that

I did wonder if would be worth your reporting as Rcpp:::LdFlags but I guess the maintainers (whose code invoked this) ought to know where is comes from.

which is a fair point. We could prefix with Rcpp:::.