Remove cc dependency for pure-rust

[pinkforest]

Ref commit message: ec3aedd

This can be removed by using Cargo environment variables in build.rs if ok removing it -

e.g. in build.rs in curve25519-dalek to gate them up further into the crate.

Also I would recommend if #5 is adopted we can gate the cc build-dependency as well cfg in-manifest.

Happy to send a PR

[jedisct1]

Could you submit a PR? That would be awesome.