Auto merge of #2946 - alexcrichton:build-script-metadata, r=brson

Always pass `-C metadata` to the compiler

If it's not otherwise available we just key it off the pkgid which should be
unique enough across compilations. This should help incremental compilation
efforts be "more incremental" across projects.

Closes #2943

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

diff --cc tests/cargo_alias_config.rs
Simple merge