Reserve Mach-O header padding on macOS so install_name_tool can relocate.

Apple's new linker (ld_prime), used on macOS 15 runners, reserves far
less header padding than the old ld. Tools like Homebrew rewrite the
install name of installed dylibs, which needs spare header space; without
it the relink fails with "updated load commands do not fit in the header".

Mirrors pyca/cryptography#14777, which fixed the same regression there.

Fixes #200.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Julian

build.rs

@@ -0,0 +1,11 @@
+fn main() {
+    // macOS 15 runners use Apple's new linker (ld_prime), which reserves
+    // far less Mach-O header padding than the old ld. Tools like Homebrew
+    // run install_name_tool on installed dylibs, which needs spare header
+    // space; without this flag the relink fails with "updated load commands
+    // do not fit in the header". See crate-py/rpds#200, and pyca/cryptography
+    // hit the same regression when its CI moved to macos-15.
+    if std::env::var("CARGO_CFG_TARGET_OS").as_deref() == Ok("macos") {
+        println!("cargo:rustc-link-arg=-Wl,-headerpad_max_install_names");
+    }
+}