I've been trying this for a few days and now noticed u-boot no longer builds with an output dir ( `make O=subdir whatever` ).

It sure took me ages to figure it was CDPATH, but CDPATH it is. I won't retry without making that "cd @" thing a subfunction (that might involve making its own completion, but that's easy compared to debugging make... I still don't know WHY it broke, just that it's CDPATH that broke it -- it decided to duplicate the subdir arg when invoking a submake so make was now left with a weird unknown target, but it obviously wouldn't say that and just failed weirdly)