Determine the location of the user and site specific ‘Makevars’ 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 file.
Section “Customizing package compilation” in the “R Installation and Administration” manual.