Brings package up to date with respect to this deprecation.
Closes #646 (though a bit of an edge case).
(Potentially?:) If a user already is providing a PKG_LIBS setting via system Makevars / environment variables, that setting will no longer be overwritten, which is presumably desirable.
Risks and drawbacks:
I haven't tested this outside my Fedora system. In particular, I haven't tested on Windows. But the Rcpp documentation does not list Windows or any other OS as an exception to this deprecation.
If people are running more-than-decade-old Rcpp versions, this may break compilation for them.
Rcpp:::LdFlags() is deprecated and produces an empty string, and hasn't been needed for long enough that there may not be loss in compatibility with current systems. Removing these Makevars settings is a simple way to address issues for some system x Rprofile combinations that I encountered (#646).
Benefits:
PKG_LIBSsetting via system Makevars / environment variables, that setting will no longer be overwritten, which is presumably desirable.Risks and drawbacks: