`~/.R/Makevars` vs. `R_HOME/etc/Makevars.site`

[coatless]

From @jimhester on rOpensci slack:

Why do you modify ~/.R/Makevars rather than the site level R_HOME/etc/Makevars.site?

• Consider generalizing the bash script to allow for site configuration...
• Test to see if a new install of R overrides the Makevars.site value.
• Add a line to the README.md about this?

[coatless]

Much better idea. Set a path to the compiler via .Renviron.