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A general description is here.
(Windows) The build of R based on ‘gcc’ 4.9.3 is now the default for R-devel on CRAN on the ‘bin/windows/base/rdevel.html’ snapshot page, and is expected to be used in R 3.3.0 (though the switch has not happened yet). This build will require recompilation of at least those packages that include C++ code, and possibly others. The CRAN binary packages still use the older toolchain, so package installs default to ‘type = "source"’. A build using the older toolchain will also be temporarily available for comparison purposes.
During the transition, the environment variable ‘R_COMPILED_BY’ will be defined to indicate which toolchain was used to compile R (and hence, which should be used to compile code in packages).