Vendor dependencies

Let's see how I like this workflow.

vendor/link-cplusplus/build.rs

@@ -0,0 +1,34 @@
+use std::env;
+use std::fs;
+use std::path::PathBuf;
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
+    let libstdcxx = cfg!(feature = "libstdc++");
+    let libcxx = cfg!(feature = "libc++");
+    let nothing = cfg!(feature = "nothing");
+
+    if nothing {
+        return;
+    }
+
+    if libstdcxx && libcxx {
+        println!(
+            "cargo:warning=-lstdc++ and -lc++ are both requested, \
+             using the platform's default"
+        );
+    }
+
+    match (libstdcxx, libcxx) {
+        (true, false) => println!("cargo:rustc-link-lib=stdc++"),
+        (false, true) => println!("cargo:rustc-link-lib=c++"),
+        (false, false) | (true, true) => {
+            // The platform's default.
+            let out_dir = env::var_os("OUT_DIR").expect("missing OUT_DIR");
+            let path = PathBuf::from(out_dir).join("dummy.cc");
+            fs::write(&path, "int rust_link_cplusplus;\n").unwrap();
+            cc::Build::new().cpp(true).file(&path).compile("link-cplusplus");
+        }
+    }
+}