Depend on BH and RcppEigen

For rstan 2.4.0, I think we should merge this branch to see if users encounter problems and then try to get it onto CRAN for rstan 3.0.0. This branch mostly just does LinkingTo for BH and RcppEigen and changes the Makefiles, but it also removes the dynamic sections of the helpfiles so that R CMD build only has to install it once (for the vignette). So we would need to tweak install_rstan.R also.