Automatically set -march, -mavx, etc flags based on CARGO_CFG_TARGET_FEATURE_* to support SIMD

[SabrinaJewson]

It would be useful if this crate read the CARGO_CFG_TARGET_ARCH and CARGO_CFG_TARGET_FEATURE environment variables to determine appropriate -march and other -m* flags to enable with CMake. For example, CARGO_CFG_TARGET_FEATURE=mmx,sse would correspond to -mmmx and -msse, CARGO_CFG_TARGET_ARCH=skylake would corresponding to -march=skylake, etc.