Auto merge of #4999 - alexcrichton:beta-next, r=alexcrichton
authorbors <bors@rust-lang.org>
Thu, 1 Feb 2018 21:15:01 +0000 (21:15 +0000)
committerbors <bors@rust-lang.org>
Thu, 1 Feb 2018 21:15:01 +0000 (21:15 +0000)
[beta] Fix `RUSTC=./relative-path` when building

This is a backport of https://github.com/rust-lang/cargo/pull/4995


Trivial merge