make buildworld: build changes only?

[Geert Hendrickx]

> make -DNOCLEAN buildworld
> 
> See /usr/src/Makefile.inc1 for all the various switches that are
> available.  You may want to set some of them permanently in
> /etc/make.conf.
> 
> Just do be aware that doing this can lead to trouble sometimes.

Ok, I think this is what I was looking for.  But what's the difference
between these: ?  

#       -DNOCLEANDIR run ${MAKE} clean, instead of ${MAKE} cleandir
#       -DNOCLEAN do not clean at all

GH