Add support for CARGO_CFG_TARGET_FEATURE

[akanalytics]

Would it be possible to add support for CARGO_CFG_TARGET_FEATURE. Typical example might be "mmx,sse"
This would allow programs to report on how optimized they are for various processors (using SIMD instructions etc).
Would be useful for chess engines written in rust :-)

[lukaslueg]

AFAICS there is currently no reliable way to do this; it's is a long-standing issue with rustc, tracked among others in rust-lang/rust#44036. Also see rust-lang/cargo#7828