Use stock Rtools toolchain by default for RTools44+

[andrjohns]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

From CmdStan 2.35 and RTools44 onwards we do not need to download/install any additional toolchains/utilities (e.g., mingw32-make). This PR updates our handling of toolchains to default to stock RTools (for rtools44 only), unless manually requested by the user setting the CMDSTANR_USE_MSYS_TOOLCHAIN environment variable.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Andrew Johnson

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[jgabry]

Great, thanks! Looks good. Should we add something about setting CMDSTANR_USE_MSYS_TOOLCHAIN to the doc before merging?

[andrjohns]

Great, thanks! Looks good. Should we add something about setting CMDSTANR_USE_MSYS_TOOLCHAIN to the doc before merging?

Ah good point! I've fleshed out the documentation in install_cmdstan, let me know if it would be better in a different place (or with more detail)

[jgabry]

Looks good, thanks!