Automatically detect features for musl

I have a similar use-case as well like in #151.
But based on the way it looks it is unlikely that this is a suitable solution. At least I couldn't get it to work.
Could you please elaborate how does this change supposed to work?

Because based on the usages this crate is always built in other crates. I.e.: In case of `openssl-sys` they are calling directly the Build in `lib.rs`. So I don't think it goes through the normal means of cargo build facilities (`build.rs`). I think this feature-flag would only be controllable in Cargo.toml if we would have the normal build procedure. (correct me if I am wrong)

Ideally I could think 2 ways to fix this problem:

- Export this feature all the way and make it possible to customize the Build section from `openssl-sys`
- Add sample test programs which can decide wether the current platform supports such facilities which are disabled by default on `musl` or `windows` (probably this one is more robust)
What do you think?