That's a good question... like `-stdlib=libstdc++`, it is supposed to cause the compiler to target the older standard library (I don't recall where I read this first; see (this Stack Overflow question)[http://stackoverflow.com/questions/19637164/c-linking-error-after-upgrading-to-mac-os-x-10-9-xcode-5-0-1] for a reference). I'm not sure it's necessary to have both; I'll try with just the stdlib flag.