This code fixes the problem:
mkdirs::
!if !exist($(OBJ))
    echo > a.tmp
# xcopy will build any missing directory structure:
    echo F | xcopy a.tmp $(OBJ)\a.tmp
    del a.tmp
    del $(OBJ)\a.tmp
!endif

But all the uses of mkdirs pass arguments that are ignored.  Should 
mkdirs process its input arguments or should they be removed from the 
callers?