Add iOS checks to build.rs

This adds some basic checks for iOS and passes them along to CMake to
build SDL properly.

I'm not sure why CMake can't figure this out, but this works!

Author: Chris--B

build.rs

@@ -66,6 +66,18 @@ fn main() {
       if arch == "aarch64" {
         cm.define("CMAKE_OSX_ARCHITECTURES", "arm64");
       }
+
+      // When building against the iOS SDK, we need to tell CMake as much
+      if env::var("CARGO_CFG_TARGET_OS").unwrap() == "ios" {
+        cm.define("CMAKE_SYSTEM_NAME", "iOS");
+        if arch == "aarch64" {
+          // All shipping iPhones are a 64-bit arm target
+          cm.define("CMAKE_OSX_SYSROOT", "iphoneos");
+        } else if arch == "x86_64" {
+          // The iPhone Simulator is an x86_64 target (TODO: Even on M1 macs?)
+          cm.define("CMAKE_OSX_SYSROOT", "iphonesimulator");
+        }
+      }
     }
 
     cm.profile("Release");