Do not always reinstall Rcpp and friends if installed

- I set a default CRAN mirror cran.rstudio.com as the fallback mirror; this will make it possible to fully automate the installation, otherwise R will pop up a menu asking for the CRAN mirror, which is not good for installing rstan on headless servers or automated testing
- I used `requireNamespace()` to check if Rcpp and friends have been installed, so that we do not have to reinstall them every single time we run this script