build.rs: add rerun-if-changed

build.rs

@@ -20,6 +20,7 @@
  */
 
 fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
     #[cfg(feature = "cli-docs")]
     {
         const MANDOC_OPTS: &[&'static str] = &["-T", "utf8", "-I", "os=Generated by mandoc(1)"];