Build script that does nothing

Eventually we will want a build script that enables Serde impls for i128
and u128. As a first step here is a build script that does nothing to
see whether we can roll this out without breaking anyone's workflow,
without having a supported feature at stake in the event that it needs
to be rolled back.

serde/build.rs

@@ -0,0 +1,39 @@
+use std::env;
+use std::process::Command;
+use std::str::{self, FromStr};
+
+fn main() {
+    let rustc = match env::var_os("RUSTC") {
+        Some(rustc) => rustc,
+        None => return,
+    };
+
+    let output = match Command::new(rustc).arg("--version").output() {
+        Ok(output) => output,
+        Err(_) => return,
+    };
+
+    let version = match str::from_utf8(&output.stdout) {
+        Ok(version) => version,
+        Err(_) => return,
+    };
+
+    let mut pieces = version.split('.');
+    if pieces.next() != Some("rustc 1") {
+        return;
+    }
+
+    let next = match pieces.next() {
+        Some(next) => next,
+        None => return,
+    };
+
+    let minor = match u32::from_str(next) {
+        Ok(minor) => minor,
+        Err(_) => return,
+    };
+
+    if minor >= 26 {
+        println!("cargo:rustc-cfg=integer128");
+    }
+}