Auto merge of #3310 - alexcrichton:more-metadata-hashing, r=brson

Apply new fingerprinting to build dir outputs

We now much more aggressively cache the output of the compiler based on feature
sets and profile configuration. Unfortunately we forgot to extend this caching
to build script output directories as well so this commit ensures that build
script outputs are cached the same way with a directory per configuration of
features and output settings.

Closes #3302

diff --cc src/cargo/ops/cargo_rustc/mod.rs
Simple merge

diff --cc tests/build-script.rs
Simple merge