Fix AudioUnit linking on iOS

On iOS, the AudioUnit framework is not (and newer were) linkable, instead one should link to AudioToolbox.

Source: https://stackoverflow.com/a/14120936

Verified by looking at simulator runtimes (this is what I could find) for version 9.3 and 10.3, on both of these AudioUnit does not contain a dylib (unlike most other frameworks). Also, in the SDK it's missing an AudioUnit.tbd file in:
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/System/Library/Frameworks/AudioUnit.framework

Author: madsmtm

build.rs

@@ -49,7 +49,23 @@ fn build(sdk_path: Option<&str>, target: &str) {
 
     #[cfg(feature = "audio_unit")]
     {
-        println!("cargo:rustc-link-lib=framework=AudioUnit");
+        // Since iOS 10.0 and macOS 10.12, all the functionality in AudioUnit
+        // moved to AudioToolbox, and the AudioUnit headers have been simple
+        // wrappers ever since.
+        if target.contains("apple-ios") {
+            // On iOS, the AudioUnit framework does not have (and never had) an
+            // actual dylib to link to, it is just a few header files.
+            // The AudioToolbox framework contains the symbols instead.
+            println!("cargo:rustc-link-lib=framework=AudioToolbox");
+        } else {
+            // On macOS, the symbols are present in the AudioToolbox framework,
+            // but only on macOS 10.12 and above.
+            //
+            // However, unlike on iOS, the AudioUnit framework on macOS
+            // contains a dylib with the desired symbols, that we can link to
+            // (in later versions just re-exports from AudioToolbox).
+            println!("cargo:rustc-link-lib=framework=AudioUnit");
+        }
         headers.push("AudioUnit/AudioUnit.h");
     }