Specify the "links" key in objc_sys

Makes it possible for downstream users to customize the linking to libobjc, see https://doc.rust-lang.org/cargo/reference/build-scripts.html#overriding-build-scripts

objc_sys/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "objc_sys"
-version = "0.0.0"
+version = "0.0.0" # Remember to update html_root_url in lib.rs
 authors = ["Mads Marquart <mads@marquart.dk>"]
 edition = "2018"
 
@@ -13,3 +13,14 @@ categories = [
 repository = "https://github.com/madsmtm/objc"
 documentation = "https://docs.rs/objc_sys/"
 license = "MIT"
+
+exclude = [
+    # Used to help developers track changes by running bindgen against
+    # different revisions of Apple's open source `objc4`.
+    "helper-scripts/*",
+]
+
+# Downstream users can customize the linking to libobjc!
+# See https://doc.rust-lang.org/cargo/reference/build-scripts.html#overriding-build-scripts
+links = "objc"
+build = "build.rs"

objc_sys/build.rs

@@ -0,0 +1,10 @@
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rustc-link-lib=dylib=objc");
+
+    // TODO: Should we vendor GNUStep's libobjc2 on non-apple targets?
+    // Check std::env::env::var("CARGO_CFG_TARGET_VENDOR").unwrap()
+    // Cargo.toml:
+    // [target.'cfg(not(target_vendor = "apple"))'.build-dependencies]
+    // cc = "1.0"
+}

objc_sys/src/lib.rs

@@ -5,15 +5,18 @@
 //! Protocol / objc_protocol is no longer a type alias of objc_object, for
 //! better type safety. Their internal representation is the same, so the
 //! functionality is just a cast away.
+//!
+//! Deprecated functions are not included since they could be removed at any
+//! macOS release, and then our code would break.
 
 // TODO: Replace `extern "C"` with `extern "C-unwind"`.
 
 #![no_std]
 #![allow(non_camel_case_types)]
 #![allow(non_upper_case_globals)]
-// Update in Cargo.toml as well.
-#![doc(html_root_url = "https://docs.rs/objc_sys/1.1.0")]
+#![doc(html_root_url = "https://docs.rs/objc_sys/0.0.0")]
 
+// TODO: Remove this and add "no-std" category to Cargo.toml
 extern crate std;
 
 use core::cell::UnsafeCell;
@@ -52,6 +55,3 @@ pub use various::*;
 ///
 /// TODO: Replace this with `extern type` to also mark it as unsized.
 type OpaqueData = PhantomData<(UnsafeCell<*const ()>, PhantomPinned)>;
-
-#[link(name = "objc", kind = "dylib")]
-extern "C" {}