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A general description is here.
(Windows) R is now built using ‘gcc’ 4.9.3. This build will require recompilation of at least those packages that include C++ code, and possibly others. A build of R-devel using the older toolchain will be temporarily available for comparison purposes.
During the transition, the environment variable ‘R_COMPILED_BY’ has been defined to indicate which toolchain was used to compile R (and hence, which should be used to compile code in packages). The ‘COMPILED_BY’ variable described below will be a permanent replacement for this.