can't run validate on OS X for 7.10 head branch!

[Carter Schonwald]

hey everyone!

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
https://gist.github.com/cartazio/ac04ab8048c9f7577d3d

thanks!
-Carter
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