Based on https://github.com/r-windows/rtools-installer.

In msys2, run make_rtools_chroot.sh.  It will produce build directory with
selected msys2 tools for building R.

Then extract the toolchain+libs (see ../toolchain_libs) into "build/rtools43".

Finally, build the installer using Inno Setup by running "iscc rtools64.iss".