error: unknown warning option '-Werror=unused-but-set-variable'; did you mean '-Werror=unused-const-variable'? [-Werror,-Wunknown-warning-option]

is the key bit, seems like apple clang bundled with xcode 6 doesn't have that option

thoughts? (do we need to do some auto conf magic to support more clang variants?)

this gist provides a bit more context

thanks!

-Carter