cocoa: mark WindowListener's window ivar unretained

ARC requires an explicitly declared ivar behind an 'assign' object
property to be __unsafe_unretained; the Xcode build stopped on it.
RARCH_UNSAFE_UNRETAINED in cocoa_defines.h spells the qualifier under
ARC and nothing under MRC, so the declaration - which the fragile
32-bit runtime needs written out - reads the same for both.

libretro-common/include/defines/cocoa_defines.h

@@ -173,6 +173,10 @@
 #endif
 
 #if __has_feature(objc_arc)
+/* Ownership qualifier for an ivar that backs an 'assign' object
+ * property: ARC requires the ivar to say it does not retain, and no
+ * other compiler knows the word. */
+#define RARCH_UNSAFE_UNRETAINED __unsafe_unretained
 #define RARCH_RETAIN(x)         (x)
 #define RARCH_RELEASE(x)        ((void)0)
 #define RARCH_AUTORELEASE(x)    ((void)0)
@@ -185,6 +189,7 @@
 #define RARCH_AUTORELEASEPOOL_BEGIN @autoreleasepool {
 #define RARCH_AUTORELEASEPOOL_END   }
 #else
+#define RARCH_UNSAFE_UNRETAINED
 #define RARCH_RETAIN(x)         [(x) retain]
 #define RARCH_RELEASE(x)        [(x) release]
 #define RARCH_AUTORELEASE(x)    [(x) autorelease]

ui/drivers/cocoa/apple_platform.h

@@ -131,8 +131,9 @@ UINavigationControllerDelegate> {
 @interface WindowListener : NSResponder RARCH_PROTO_NSWINDOWDELEGATE
 {
 	/* Declared, not synthesized: the fragile 32-bit runtime GCC 4.0
-	 * targets cannot add ivars for a property on its own. */
-	NSWindow *_window;
+	 * targets cannot add ivars for a property on its own.  Unretained,
+	 * as the assign property it backs requires under ARC. */
+	RARCH_UNSAFE_UNRETAINED NSWindow *_window;
 }
 /* assign (not retain) - WindowListener is a delegate; the window owns it, not vice versa */
 @property (nonatomic, assign) NSWindow *window;