Implement build script

.gitignore

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+/Cargo.lock
+/target

Cargo.toml

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+[package]
+name = "link-cplusplus"
+version = "0.0.0"
+authors = ["David Tolnay <dtolnay@gmail.com>"]
+edition = "2018"
+
+[build-dependencies]
+cc = "1.0"
+
+[features]
+default = [] # automatic
+libstdcxx = [] # force libstdc++
+libcxx = [] # force libc++
+nothing = [] # link nothing, determined somewhere else

build.rs

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+fn main() {
+    let libstdcxx = cfg!(feature = "libstdcxx");
+    let libcxx = cfg!(feature = "libcxx");
+    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.
+            cc::Build::new().cpp(true).compile("link-cplusplus");
+        }
+    }
+}

src/lib.rs

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+// woohoo