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?

Empieza revisando build.rs y las definiciones de features que habilitan libc y errno; después, compara el grafo de dependencias informado por cargo metadata con el comportamiento de cargo vendor-filterer. El problema se resuelve cuando la configuración de dependencias predeterminada se representa de forma coherente sin una activación exclusiva de build.rs y cargo vendor-filterer deja de fallar.

Escrito por el modelo de indexación a partir del texto del issue.