haku2: make build.rs spit out unknown targets

crates/haku2/build.rs

@@ -28,7 +28,10 @@ fn main() -> Result<(), Box<dyn Error>> {
 
     let target = match &target[..] {
         "x86_64-unknown-linux-gnu" => "x86_64-linux-gnu",
-        _ => &target,
+        other => {
+            eprintln!("warning: unknown target {other:?}");
+            &target
+        }
     };
 
     let output = Command::new("zig")