system variable within R

To let R know about the compiler, we need to modify the ~/.Renviron. You may need to create the file by running in Terminal (Applications -> Utilities):

touch ~/.Renviron

Then, inside of ~/.Renviron add:

PATH="/usr/local/clang7/bin:${PATH}"

See http://thecoatlessprofessor.com/programming/r-compiler-tools-for-rcpp-on-macos/