Merge pull request #1459 from mime29:macrowarning

Updated dispatch_ macro in order to avoid redefinition when included as Pod

SDWebImage/SDWebImageCompat.h

@@ -99,11 +99,13 @@ typedef void(^SDWebImageNoParamsBlock)();
 
 extern NSString *const SDWebImageErrorDomain;
 
+#ifndef dispatch_main_async_safe
 #define dispatch_main_async_safe(block)\
     if (strcmp(dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL), dispatch_queue_get_label(dispatch_get_main_queue())) == 0) {\
         block();\
     } else {\
         dispatch_async(dispatch_get_main_queue(), block);\
     }
+#endif
 
 static int64_t kAsyncTestTimeout = 5;