build-dependencies not captured on crates.io

[sjmackenzie]

build-dependencies e.g. listed here: https://github.com/thehydroimpulse/nanomsg.rs/blob/master/nanomsg_sys/Cargo.toml#L26-L29

are not captured here: https://github.com/rust-lang/crates.io-index/blob/master/na/no/nanomsg-sys#L10

I'm very curious about the design decisions for build-dependencies. So quite possibly on the user end these dependencies need to be injected into a derivation.