stop enabling dependencies in build.rs by default?

(I'm basically using issues as a communication mechanism here)

See https://github.com/coreos/cargo-vendor-filterer/issues/71

TL;DR when rustix 0.37 came along with https://github.com/bytecodealliance/rustix/commit/a2c6c7a8b7adcf46592b7519ba90ecece18b4bd6 this ended up enabling `libc` by default and that pulled in the `errno` dependency by default.

However, `cargo metadata` doesn't see this dependency, it's only enabled via `build.rs`.

And that ends up breaking `cargo vendor-filterer`.

Since in reality, libc is enabled by default, ISTM we can rework the features and build scripts here right?

Start by reviewing build.rs and the feature definitions that enable libc and errno, then compare the dependency graph reported by cargo metadata with cargo vendor-filterer behavior. The issue is resolved when the default dependency configuration is represented consistently without build.rs-only activation and cargo vendor-filterer no longer breaks.

Written by the indexing model from the issue text.