Trigger rebuild if a curl source file has changed. (#407)

Author: ehuss

curl-sys/build.rs

@@ -4,6 +4,7 @@ use std::path::{Path, PathBuf};
 use std::process::Command;
 
 fn main() {
+    println!("cargo:rerun-if-changed=curl");
     let host = env::var("HOST").unwrap();
     let target = env::var("TARGET").unwrap();
     let windows = target.contains("windows");