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A general description is here.
Old make targets ‘rsync-extsoft’ and 32-bit ones that are no longer needed have been removed.
Default builds (including for packages) no longer select C99. Thus the C standard used is the default for the compiler, which for the toolchain in ‘Rtools43’ is C17. (This is consistent with Unix builds.)
The makefiles and installer scripts for Windows have been tailored to ‘Rtools43’, an update of the ‘Rtools42’ toolchain. It is based on ‘gcc’ 12 and newer versions of MinGW-W64, binutils and libraries. At this time R-devel can still be built using Rtools42 without changes, but when R-devel is installed via the installer, it will by default look for Rtools43. Pre-release builds of Rtools43 are available from <URL: https://www.r-project.org/nosvn/winutf8/ucrt3/>.