Don't try and build the SIMD version on OS X. And especially on Xcode 5.
Also, the SIMD version is reliably 10% slower on Sandy Bridge-era CPUs (confirmed on multiple i7 2600Ks), which is anecdotally at least most Intel Macs currently in circulation.
ETA: to clarify, that's 10% slower than the normal non-SIMD build, not 10% slower than the speedups JD was claiming
