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?

まず build.rs と libc および errno を有効にする feature 定義を確認し、次に cargo metadata が報告する依存関係グラフと cargo vendor-filterer の動作を比較します。デフォルトの依存関係構成が build.rs のみによる有効化なしで一貫して表現され、cargo vendor-filterer が失敗しなくなれば、問題は解決です。

索引モデルが issue の本文から書いたものです。