When you installed Rcpp above, it installed a .tgz, which is a built binary, not a "source" tarball, which will make the program compile and link to your specific setup, in case it's different than that from CRAN. So if you definitely believe you installed from source, then see other answers. Otherwise, try by source again.
It's possible that the version of libc++ bundled by R is not compatible with El Capitan. Are you able to update the version of macOS you're using on your machine?