Switch from `rustc_version` to `autocfg`

This switches the `openssl-sys` crate from using `rustc_version` as a
crate to check the version of rustc to using `autocfg`. While
functionally the same this has a few advantages:

* The `autocfg` crate has fewer dependencies and compiles faster
* If the `semver` crate has the `serde` feature activated, turns out
  `openssl-sys` gets compiled quite late in the dependency graph which
  can push back further C compilations. This is due to the slower
  compilation time of `serde` itself.
* The `autocfg` crate I believe is a bit more robust in terms of being
  flexible with the output of rustc itself.