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A general description is here.
(Windows) A ‘make’ and ‘R CMD config’ variable named ‘COMPILED_BY’ has been added. This indicates which toolchain was used to compile R (and hence, which should be used to compile code in packages).
(Windows) An experimental build of R based on ‘gcc’ 4.9.3 is now available on CRAN on the ‘bin/windows/base/rdevel.html’ snapshot page. 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"’.
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). The ‘COMPILED_BY’ variable described below will be a permanent replacement for this.