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A general description is here.
The supported toolchain has changed since the one used for R 2.12.0 to 2.14.x: see the‘R Installation and Administration manual’. Compiled code (except DLLs) may be incompatible with previous toolchains (and compiled C++ code almost certainly will be).
This is a ‘multilib’ toolchain: there is a single set of tools in the ‘bin’ directory and which sub-architecture is selected by a flag: in most cases ‘-m32’ _vs_ ‘-m64’.