Manually overriding SIMD vectorization type

Also, the installer may occasionally want to refrain from using a feature that this machine possesses, but other machines that plan to use the same library don't. For instance, the installation machine might have SSE3, but the installer wants to use the same library on machines that possess only SSE2, and so the installer wants to use only SSE2, even though this will make the library sub-optimal on the machine possessing SSE3.