allow "unexpected_cfgs" in macOS OpenGL code

Author: httnn

src/gl/macos.rs

@@ -1,3 +1,7 @@
+// This is required because the objc crate is causing a lot of warnings: https://github.com/SSheldon/rust-objc/issues/125
+// Eventually we should migrate to the objc2 crate and remove this.
+#![allow(unexpected_cfgs)]
+
 use std::ffi::c_void;
 use std::str::FromStr;