>  I've seen examples of the i5-4670K with an extmodel 1 through 4

ah interesting! Whatever the meaning of `extmodel` is though my point still stands - unless you have a counterexample of a Haswell Mavericks machine successfully compiling with SSSE3, or a non-Haswell non-Mavericks machine failing to compile with SSSE3 the test should be 

    if build.without?('ssse3') || MacOS.version <= :snow_leopard || (MacOS.version >= Mavericks && Hardware::CPU.family == :haswell)