stan() can't find g++ if rtools not on c drive (windows)

Summary:

I have Rtools installed on my e drive.
executing the stan(..) method from RStudio fails

Description:

executing the following line at the Rsudio prompt

hlm = stan(model_name="Hierarchical Linear Model", model_code = hcode, data=dat , iter = 2000, chains = 2, verbose = FALSE)

gives

c:/Rtools/mingw_64/bin/g++: not found

system('where g++')
E:\Rtools\gcc-4.6.3\bin\g++.exe

Reproducible Steps:

Please report steps to reproduce the issue. If it's not possible to reproduce, please include a description of how you discovered the issue.

If you have a reproducible example, please include it.

Current Output:

If applicable, any relevant output from RStan.

Expected Output:

If applicable, the output you expected from RStan.

RStan Version:

packageVersion("rstan")
[1] ‘2.12.1

R Version:

version.string R version 3.3.1 (2016-06-21)
nickname Bug in Your Hair

Operating System:

Windows 7

Start by reproducing the failure from the RStudio prompt with stan() and compare the configured compiler path with system('where g++') and the reported Rtools locations. Trace how stan() selects g++ on Windows; done means the example can find the E:\Rtools compiler and complete compilation when Rtools is not installed on C:.