If the clang compiler doesn't work for 1.9.3 that's fine, but it shouldn't be so oblivious to use it by default. Forcing gcc is the only way this will work, so a warning on non-compliant compilers is better than barfing on a C error.
@tadman, clang does work on 1.9.3. When a new Xcode released, it included a new clang that introduced new kinds of warnings, which Ruby treats as errors by default. Ruby tripped one of the new warnings, which made a new error appear. Nothing's being oblivious here. We'll get it worked out!