Auto merge of #3243 - alexcrichton:target-feature, r=brson

Expose rustc cfg values to build scripts

This commit is Cargo's portion of the implementation of [RFC 1721] where it will
expose values printed by `rustc --print cfg` to build scripts.

[RFC 1721]: https://github.com/rust-lang/rfcs/blob/master/text/1721-crt-static.md

This will in turn be used to communicate features like `-C
target-feature=+crt-static` which can be used to compile objects for statically
linking against the msvcrt on MSVC.