Vendor dependencies

Let's see how I like this workflow.

vendor/link-cplusplus/README.md

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+`-lstdc++` or `-lc++`
+=====================
+
+[<img alt="github" src="https://img.shields.io/badge/github-dtolnay/link--cplusplus-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/link-cplusplus)
+[<img alt="crates.io" src="https://img.shields.io/crates/v/link-cplusplus.svg?style=for-the-badge&color=fc8d62&logo=rust" height="20">](https://crates.io/crates/link-cplusplus)
+[<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-link--cplusplus-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" height="20">](https://docs.rs/link-cplusplus)
+[<img alt="build status" src="https://img.shields.io/github/actions/workflow/status/dtolnay/link-cplusplus/ci.yml?branch=master&style=for-the-badge" height="20">](https://github.com/dtolnay/link-cplusplus/actions?query=branch%3Amaster)
+
+This crate exists for the purpose of passing `-lstdc++` or `-lc++` to the
+linker, while making it possible for an application to make that choice on
+behalf of its library dependencies.
+
+Without this crate, a library would need to:
+
+- pick one or the other to link, with no way for downstream applications to
+  override the choice;
+- or link neither and require an explicit link flag provided by downstream
+  applications even if they would be fine with a default choice;
+
+neither of which are good experiences.
+
+<br>
+
+## Options
+
+An application or library that is fine with either of libstdc++ or libc++ being
+linked, whichever is the platform's default, should use the following in
+Cargo.toml:
+
+```toml
+[dependencies]
+link-cplusplus = "1.0"
+```
+
+An application that wants a particular one or the other linked should use:
+
+```toml
+[dependencies]
+link-cplusplus = { version = "1.0", features = ["libstdc++"] }
+
+# or
+
+link-cplusplus = { version = "1.0", features = ["libc++"] }
+```
+
+An application that wants to handle its own more complicated logic for link
+flags from its build script can make this crate do nothing by using:
+
+```toml
+[dependencies]
+link-cplusplus = { version = "1.0", features = ["nothing"] }
+```
+
+Lastly, make sure to add an explicit `extern crate` dependency to your crate
+root, since the link-cplusplus crate will be otherwise unused and its link flags
+dropped.
+
+```rust
+// src/lib.rs
+
+extern crate link_cplusplus;
+```
+
+<br>
+
+#### License
+
+<sup>
+Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
+2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
+</sup>
+
+<br>
+
+<sub>
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in this project by you, as defined in the Apache-2.0 license,
+shall be dual licensed as above, without any additional terms or conditions.
+</sub>