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You probably shouldn't be passing --build at all; it's normally only
needed for cross-compiling. If you do set it, it should be something
like x86_64-apple-darwin12.

The -O20 is hardcoded into the configure script, and probably shouldn't
be, since you could be using clang (on many platforms) and it rejects
it. However with the build system type set correctly it looks like you
won't see the problem, since -O20 is not used for darwin.

- Josh