Auto merge of #3664 - pwoolcoc:issue-3391, r=alexcrichton

Assume that a `build.rs` file is a build script

If cargo sees a `build.rs` file in the same directory as the current
`Cargo.toml`, it will assume that the `build.rs` file is a build script,
unless there is `build = false` in the  `Cargo.toml` file.

Closes #3391