Automatically detect features for musl

[havasd]

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?