Auto merge of #3689 - ms705:master, r=alexcrichton

More intuitive CARGO_INCREMENTAL semantics

Currently, the mere presence of a `CARGO_INCREMENTAL` variable in the environment causes incremental compilation to be enabled. This has the very counterintuitive effect that `CARGO_INCREMENTAL=0` and even `CARGO_INCREMENTAL=` mean incremental compilation is *on*.

This PR brings the semantics in line with how they are defined in the tests (cf. [tests/build.rs:45](https://github.com/rust-lang/cargo/blob/master/tests/build.rs#L45)), and in [public-facing documentation](https://internals.rust-lang.org/t/incremental-compilation-beta/4721).

See also [rust#39773](https://github.com/rust-lang/rust/issues/39773) for an example of this causing confusion in the wild.