@goedman @bob-carpenter 

I think having the C++11 flag imply `CFLAGS += -stdlib=libc++ -std=c++11` is not quite right because we also want to support the c++11 standard under g++ (>= 4.7). It is technically possible to use LLVM's version of the standard c++ library with g++ but that is sort of an academic exercise and essentially everyone using g++ would be using GCC's version of the standard c++ library. 

So, I think if someone wants to use the C++11 standard, they should just add the appropriate flags locally to their make/os_{win,mac,linux} file. Or maybe we should source another (possibly empty) file under make/ called user_options or something.