Auto merge of #3929 - Xion:master, r=alexcrichton

Allow cargo:rustc-env in build scripts

This is an attempt to address issue #2875. Basically, I'm trying to allow build scripts to produce`cargo:rustc-env=FOO=foo` lines in their output. These are then translated to `env!()`-friendly env. vars that rustc is run with.

diff --cc src/cargo/ops/cargo_compile.rs
Simple merge