Fails to build on Windows

Hi,

I apologise for how long it has taken for me to file this issue, but... This crate fails to build on Windows. I've tried both in CI on Travis, and in a local build both in git-bash and from Windows cmd prompt. In all cases, the failure comes down to the fact that you attempt to build nettle by means of running its configure script which is a *NIX shell script - not terribly easy to run on Windows.

I have abstracted the code I wrote as a proof-of-concept in rustup and pushed it to a repository here on Gitlab (https://gitlab.com/kinnison/simple-nettle-test) this builds and runs fine on my Linux boxes, but not on any of the Windows systems I have access to.

If you want me to try different updates to nettle-src then I can add more [patch.crates-io] entries to Cargo.toml easily.

Thanks,

Daniel.