Update Rtools detection for Rtools 3.3

[jjallaire]

Need to do two things:

• Add Rtools\bin to the PATH (you don't add the gcc directories like you used to)
• Set an RTOOLS environment variable to the location of Rtools (e.g. C:/Rtools33/). Note that forward slashes and the trailing slash are required.

Similar work was done for:

• Rcpp: Use Rtools 3.3 for compilation under R 3.2 RcppCore/Rcpp#274
• RStudio: https://github.com/rstudio/rstudio/blob/45df6cbb483cc03471d8094b323984d7576b7640/src/cpp/core/r_util/RToolsInfo.cpp#L109-L120

[dtenenba]

I guess you don't want to do this anymore (at least not yet); sounds like Rtools33 is going to roll back to using gcc 4.6.3.

[lock]

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