Modify the clean target.  If the following are invoked:

   (1)  make clean USE_NEW_LAYOUT=yes
        -> this will only blow away objects generated during the
           build which exist under $AVATAR_ROOT/../obj

   (2)  make distclean USE_NEW_LAYOUT=yes
        -> this will first do (1), and then
           blow away all sources under $AVATAR_ROOT/../extra-src
           which were checked out with 'make checkout USE_NEW_LAYOUT=yes'

If USE_NEW_LAYOUT is not specified, then the old
behavior of the clean target occurs.  This will allow us
to conditionally try out the new clean target, until we decide
to fully switch over to the new behavior.