Auto merge of #4234 - michaelwoerister:smarter-cargo-incremental-2, r=alexcrichton

Auto merge of #4234 - michaelwoerister:smarter-cargo-incremental-2, r=alexcrichton

Set -Ccodegen-units=N for non-path dependencies when CARGO_INCREMENTAL is set.

This PR tweaks compilation for non-path dependencies when CARGO_INCREMENTAL is set. Before, `cargo` would compile such dependencies "just" non-incrementally while, with this PR, it will also set the number of CGUs to the number of CPU cores. Otherwise compiling those dependencies might take significantly longer (because of more expensive optimizations and less multi-threading) than compiling them incrementally -- which is what we wanted to avoid in the first place.