"Building R and R packages" section needs updated to reflect the changes with Rtools42

[yutannihilation]

I was trying to contribute to R-devel by following the guide, but it turned out we cannot use r-windows/r-base for R >=4.2.

https://github.com/r-windows/r-base/pull/34#issuecomment-1001213439

I'm really hoping the situation will be improved, so I have no idea how the guide should be updated at the moment.

[mmaechler]

Yes, building R from the sources is currently (back to) be covered entirely by the R Installation and Administration manual the source of which is part of the R sources, hence maintained by R core.

Building on Windows has its own important chapter Installing R under Windows, which has been updated for the Rtools42 (Windows Rtools for R >= 4.2.0) extensively.

The Rtools42 being relatively new, it can well have things missing, or not easy enough to understand. For the time being, the R-devel mailing list may be the best place to ask for clarifications.

[hturner]

Fixed by #74