Provide better compile support for pre AVX and specific SIMD level compiling

[JimHokanson]

I've found out that any pre AVX processors will cause the current code to crash. I've created a hack but the code needs to be cleaned up. Additionally, it would be nice to allow compiling to target a specific SIMD level, specifically to only allow AVX (and not AVX2). Currently you can only enable or disable both simultaneously.

On a related note, I've also wanted to have a runtime flag for testing the various pathways, rather than needing to recompile.