Compile time check for blocks that is of no use when min. target < 4.0 anyway

blocks_do_not_work_like_this_irl.m

#ifdef NS_BLOCKS_AVAILABLE
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
#endif    

        // background stuff
#ifdef NS_BLOCKS_AVAILABLE
        dispatch_sync(dispatch_get_main_queue(), ^(void) {
#endif    
             // main thread stuff


#ifdef NS_BLOCKS_AVAILABLE
        });
});
#endif

Problem: checking against NS_BLOCKS_AVAILABLE at compile time will of course work on a 4.0 or 5.0 SDK app, this is something that needs to be checked at runtime instead - see https://gist.github.com/1478748