Auto merge of #36853 - TimNN:rustbuild-out-of-tree, r=alexcrichton
Auto merge of #36853 - TimNN:rustbuild-out-of-tree, r=alexcrichton

fix out-of-tree rustbuild

See https://github.com/rust-lang/rust/pull/36456#issuecomment-250589906

r? @alexcrichton

GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust