Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/build.rs

@@ -0,0 +1,55 @@
+use std::env;
+use std::path::Path;
+use std::process::Command;
+
+fn main() {
+    cc::Build::new()
+        .file("src/cxx.cc")
+        .cpp(true)
+        .cpp_link_stdlib(None) // linked via link-cplusplus crate
+        .flag_if_supported(cxxbridge_flags::STD)
+        .warnings_into_errors(cfg!(deny_warnings))
+        .compile("cxxbridge1");
+
+    println!("cargo:rerun-if-changed=src/cxx.cc");
+    println!("cargo:rerun-if-changed=include/cxx.h");
+    println!("cargo:rustc-cfg=built_with_cargo");
+
+    if let Some(manifest_dir) = env::var_os("CARGO_MANIFEST_DIR") {
+        let cxx_h = Path::new(&manifest_dir).join("include").join("cxx.h");
+        println!("cargo:HEADER={}", cxx_h.to_string_lossy());
+    }
+
+    if let Some(rustc) = rustc_version() {
+        if rustc.minor < 48 {
+            println!("cargo:warning=The cxx crate requires a rustc version 1.48.0 or newer.");
+            println!(
+                "cargo:warning=You appear to be building with: {}",
+                rustc.version,
+            );
+        }
+
+        if rustc.minor < 52 {
+            // #![deny(unsafe_op_in_unsafe_fn)].
+            // https://github.com/rust-lang/rust/issues/71668
+            println!("cargo:rustc-cfg=no_unsafe_op_in_unsafe_fn_lint");
+        }
+    }
+}
+
+struct RustVersion {
+    version: String,
+    minor: u32,
+}
+
+fn rustc_version() -> Option<RustVersion> {
+    let rustc = env::var_os("RUSTC")?;
+    let output = Command::new(rustc).arg("--version").output().ok()?;
+    let version = String::from_utf8(output.stdout).ok()?;
+    let mut pieces = version.split('.');
+    if pieces.next() != Some("rustc 1") {
+        return None;
+    }
+    let minor = pieces.next()?.parse().ok()?;
+    Some(RustVersion { version, minor })
+}