NSSpeechSynthesizerDelegate: define protocol properly

Modern applications require NSSpeechSynthesizerDelegate to be an actual
@protocol in order to conform to it. Here, we allow it to be one if we
are on an Objective-C runtime that supports it.

Headers/AppKit/NSSpeechSynthesizer.h

@@ -151,8 +151,13 @@ extern NSString *NSSpeechDictionaryEntryPhonemes;
 - (NSString *) phonemesFromText: (NSString *)text;
 @end
 
-// Speech synthesizer delegate informal protocol...
+@protocol NSSpeechSynthesizerDelegate <NSObject>
+#if GS_PROTOCOLS_HAVE_OPTIONAL
+@optional
+#else
+@end
 @interface NSObject (NSSpeechSynthesizerDelegate)
+#endif
 - (void)speechSynthesizer: (NSSpeechSynthesizer *)sender
  didEncounterErrorAtIndex: (NSUInteger)index 
                  ofString: (NSString *)string