Both Apple and LLVM Clang want -msse4.2 even when only SSE4.1 is used. Sidestep it because we don't know how it will affect some of the lower end Atoms.
84877:/usr/include/clang/3.5.0/include/nmmintrin.h:28:2: error: "SSE4.2 instruction set not enabled"
84878:#error "SSE4.2 instruction set not enabled"
84880:rijndael-simd.cpp:466:26: error: use of undeclared identifier '_mm_extract_epi32'; did you mean '_mm_extract_epi16'?
84887:rijndael-simd.cpp:480:11: error: use of undeclared identifier '_mm_insert_epi32'; did you mean '_mm_insert_epi16'?
84894:rijndael-simd.cpp:485:11: error: use of undeclared identifier '_mm_insert_epi32'; did you mean '_mm_insert_epi16'?
...
getauxval() is the recommended way to determine features on Linux. Its likely less expensive than CPU probing for SIGILLs. We gave up portability, but some gained stability
Jeffrey Walton