Rerun the Metal build script when DEVELOPER_DIR changes

The script finds the Metal compiler through xcrun, which follows
DEVELOPER_DIR. A build that ran while xcrun pointed at the Command Line Tools
embedded empty placeholder metallibs, and setting DEVELOPER_DIR afterwards
changed nothing until the crate was cleaned, because cargo did not know the
script depends on that variable.

Author: ualtinok

crates/synapse-engine-owned/build.rs

@@ -13,6 +13,11 @@ fn main() {
     ] {
         println!("cargo:rerun-if-changed={source}");
     }
+    // The Metal toolchain is located through xcrun, which follows DEVELOPER_DIR.
+    // Without this line, a build that ran while xcrun pointed at the Command
+    // Line Tools keeps its empty placeholder metallibs after DEVELOPER_DIR is
+    // set, until the crate is cleaned.
+    println!("cargo:rerun-if-env-changed=DEVELOPER_DIR");
 
     if std::env::var("CARGO_CFG_TARGET_OS").as_deref() != Ok("macos") {
         return;