Not having touched cc wrapper code, I want to make sure after adding `HOMEBREW_CCCFG` symbol it should be handled at https://github.com/mxcl/homebrew/blob/master/Library/ENV/4.3/cc#L66-L73, right?

But back to the current implementation, I think it is more clearer to expose those information here. Also, those flags are specific to C++ compiler command. They cannot be reliably used as CXXFLAGS or LDFLAGS, especially LDFLAGS is shared among different compilers/linker and may cause problem (e.g., building open-mpi when gfortran is in use). So adding those flags to $CXX is the only reliable way to specify C++11 mode, which I think is safe here.