Only build if there is a change.

wordlang/build.rs

@@ -63,6 +63,9 @@ fn read_mappings() -> Result<(HashMap<String, Vec<String>>, HashMap<String, Stri
 }
 
 fn main() -> Result<(), Box<dyn Error>> {
+    println!("cargo:rerun-if-changed=iso-639-3-macrolanguages.tab");
+    println!("cargo:rerun-if-changed=languages.txt");
+
     let languages = read_languages()?;
     let (m_to_l, l_to_m) = read_mappings()?;