Explore alternatives to RTools

[WardBrian]

We are still installing Rtools 40, which is about to be five versions out of date. Using a newer RTools is difficult, in part because they no longer provide stable download links. I also feel like using an R project at all is kind of strange

One alternative I think would be reasonable is the https://github.com/skeeto/w64devkit/ project. The build process is very transparent, the tools are up to date, and the download is relatively small. I need to actually verify that cmdstan builds once it is used, but I expect it will.

[WardBrian]

Also consider: https://www.winlibs.com/