Avoid compiler warnings when we do not have real blocks

Source/NSFileCoordinator.m

@@ -153,7 +153,10 @@ static NSMutableDictionary *__presenterIdDict = nil;
   
   while((obj = [en nextObject]) != nil)
     {
-      NSBlockOperation *op = [NSBlockOperation blockOperationWithBlock: accessor];
+      NSBlockOperation *op;
+
+      op = [NSBlockOperation
+	blockOperationWithBlock: (GSBlockOperationBlock)accessor];
       [queue addOperation: op];
     }
 }

Source/NSItemProvider.m

@@ -152,7 +152,7 @@ NSString * const NSItemProviderPreferredImageSizeKey = @"NSItemProviderPreferred
 
 - (NSItemProviderLoadHandler) previewImageHandler
 {
-  return nil;
+  return (NSItemProviderLoadHandler)0;
 }
 
 - (void) setPreviewImageHandler: (NSItemProviderLoadHandler) previewImageHandler

Source/NSSortDescriptor.m

@@ -223,7 +223,7 @@ static BOOL     initialized = NO;
 
       ASSIGN(_key, key);
       _ascending = ascending;
-      ASSIGN(_comparator, cmptr);
+      ASSIGN(_comparator, (id)cmptr);
 
       return self;
     }

Source/NSTimer.m

@@ -150,7 +150,7 @@ static Class	NSDate_class;
                           repeats: (BOOL)repeats
                             block: (GSTimerBlock)block
 {
-  ASSIGN(_block, block);
+  ASSIGN(_block, (id)block);
   return [self initWithFireDate: date
                        interval: interval
                          target: nil