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I guess it's using SSE2. It automatically sets its build configuration at compile time based on the compiler environment.
It looks like it has some SSE2 detection at runtime, but it's hard to tell if it actually uses it to fall back if it's not available. I do want the release builds to have SSE2. SSE2 has been around since the first Pentium 4. A Pentium 3 or older would be so slow, you'd be wasting your electricity trying to generate on it anyway.
I've got some older machines (for the windows client and linux clients) to test with that don't support SSE2. Mainly if you try to run them, the program just crashes when I tried some of the experimental builds here, but I'll be glad to test some future official builds to see if the "detect SSE2" part works or the program goes belly up.