To build binary packages on windows, Rtools (found at
https://CRAN.R-project.org/bin/windows/Rtools/) needs to be on
the path. The default installation process does not add it, so this
script finds it (looking first on the path, then in the registry).
It also checks that the version of rtools matches the version of R.
has_rtools()
determines if Rtools is installed, caching the results.
Afterward, run rtools_path()
to find out where it's installed.