Also rebuild if build.rs itself changes.

We already have all the meta/**.py as dependencies.

lib/cretonne/build.rs

@@ -23,6 +23,11 @@ fn main() {
     let cur_dir = env::current_dir().expect("Can't access current working directory");
     let crate_dir = cur_dir.as_path();
 
+    // Make sure we rebuild is this build script changes.
+    // I guess that won't happen if you have non-UTF8 bytes in your path names.
+    // The `build.py` script prints out its own dependencies.
+    println!("cargo:rerun-if-changed={}", crate_dir.join("build.rs").to_string_lossy());
+
     // Scripts are in `$crate_dir/meta`.
     let meta_dir = crate_dir.join("meta");
     let build_script = meta_dir.join("build.py");