User and Site Compilation Variables
Determine the location of the user and site specific
files for customizing package compilation.
Package maintainers can use these functions to employ user and site
specific compilation settings also for compilations not using R's
mechanisms (in particular, custom compilations in subdirectories of
src), e.g., by adding configure code calling R with
cat(tools::makevars_site()), and if non-empty passing this with
-f to custom Make invocations.
A character string with the path to the user or site specific
Makevars file, or an empty character vector if there is no such
Section “Customizing package compilation” in the “R Installation and Administration” manual.