Backported libraries with the Rtools35 gcc-4.9.3 legacy toolchain.
As of version 4.0 (April 2020), R on Windows switched to a new build system called rtools40, which includes at a toolchain based on GCC-8. This new system enables us to automatically build and distribute external C/C++ libs needed for building R packages.
However for legacy users we also try to keep supporting R for Windows versions 3.3-3.6 which use the gcc-4.9.3 toolchain from Rtools35. Because some C/C++ libraries are ABI sensitive, these need to be compiled with the same toolchain that is used to compile the R package. This is what the binaries in rtools-backports are used for.
The CI in this repository uses the regular rtools40 build environment and dependencies from the rtools-packages repo. However for packages in rtools-backports, we manually override the build scripts to use compilers from Rtools35 by setting CC and CXX environment variables in the PKGBUILD files. Thereby the libs get compiled with gcc-4.9.3 and the resulting binaries can be used to build R packages with the old toolchain.
Rtools35 did not include a package manager so these libs are not automatically deployed anywhere. Upon success, you can download the binaries from appveyor artifacts or bintray. We manually test them (by building the R package), and if they seem OK we may host a copy of the binaries in the rwinlib organization on Github.
This is all a bit hacky, we try to keep it working on a best effort basis. Users are very much recommended to upgrade to R 4.0 or newer!