Fork app in main.m between old codebase for iOS5/6 and new for iOS7+

main.m

NSUInteger DeviceSystemMajorVersion();
NSUInteger DeviceSystemMajorVersion() {
    static NSUInteger _deviceSystemMajorVersion = -1;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _deviceSystemMajorVersion = [[[[[UIDevice currentDevice] systemVersion] componentsSeparatedByString:@"."] objectAtIndex:0] intValue];
    });
    return _deviceSystemMajorVersion;
}

#define OLDSCHOOL (DeviceSystemMajorVersion() < 7)

int main(int argc, char *argv[]) {

    if (OLDSCHOOL) {
        NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
        int retVal = UIApplicationMain(argc, argv, nil, @"AppDelegate");
        [pool release];
        return retVal;
    }
    else
    {
        @autoreleasepool {
            return UIApplicationMain(argc, argv, nil, @"FlatAppDelegate");
        }

    }
    
}