With Xcode I can't get Embree to work in 64 bits (wth/without ispc). Basic = operations use avx commands that my machine has not. Crash screenshots are attached. In 32bits all is fine. Embree 2.3.3, Xcode 4.6.2. icc 13.0, OSX 10.7.
Embree uses a cmake-based build infrastructure which takes care that only special code paths are build with AVX/AVX2/.. instructions, and all "surrounding" code uses only simple SSE instructions. The special AVX/AVX2 code paths are only executed at run-time when your machine supports the specific ISA extensions. Embree does not support the Xcode build environment directly so I guess you would have to somehow emulate the cmake infrastructure/rules within Xcode.