Auto merge of #1086 - IsaacWoods:master, r=alexcrichton

Revisit work on cvoid

At the recommendation of @SimonSapin [here](https://github.com/rust-lang/rust/issues/53856#issuecomment-422932721), I have revisited the build script to check whether `core::ffi::c_void` resolves, instead of relying on a particular `rustc` version. This allows use on `1.30.x` builds with `core::ffi::c_void`.

I also noticed that `c_void` is defined twice in this crate, once in `lib.rs` and again in `switch.rs`. This instead defines `c_void` for every target except `wasm32`. As far as I can tell, this shouldn't actually change functionality on any existing targets.