Version 2.1 WIP

Summary

-Default build is demoted to SSE2, as most CPU's still in use by gamers do not support AVX at all (Phenom, Core and early Core i.
-AVX and AVX2 are install options
-12on7 updated to latest to include some leak fixes.

Note: A future release of d912pxy upstream will inherit the SIMD changes and improve on the installer to automatically detect the SIMD capabilities.