Re-export core::ffi::c_void if it exists

[IsaacWoods]

This is the second part of the implementation of RFC 2521, replacing the definition of c_void in libc with a re-export of the type from core::ffi::c_void on builds it exists for.

This uses the re-export for rustc version 1.31.0 or greater, as 1.30.x was the current nightly when the PR for the changes to libcore and libstd was merged, so I'm assuming the first nightly they will appear in will be 1.31.0; is this acceptable?

cc rust-lang/rust#53856

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[alexcrichton]

Thanks! The rustc_version crate is a bit heavyweight for our purposes though. Could this perhaps use something a bit more lightweight like this?

[IsaacWoods]

Done :)

I think the one Travis failure (stable, TARGET=aarch64-linux-android) is spurious, as the rest pass and it just appears to be timing out. Could someone issue a retry?

[alexcrichton]

@bors: r+

[bors]

📌 Commit 79c80c4 has been approved by alexcrichton

[bors]

⌛ Testing commit 79c80c4 with merge d13f346...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing d13f346 to master...