several changes to runRcppDepends

- set CC and CXX as there is a braindead stupid Ubuntu issue with autoconf
  and llvm where the former insists on using the latter, and always fails
- remove old repo cache files
- include roxygen2 which passes now
- check if package is already installed; if so skip re-installation
- copy tarball from local CRANberries mirror (if mirror exists, and has file)