nbmake: don't know how to make /usr/src/sys/sys/properties.h. Stop

I've tried 4 times to cvs update /usr/src against current and get a
clean compile of GENERIC.MP with the command sequence

cd /usr/src
cvs update -dPA
./build.sh -u tools kernel=GENERIC.MP

Because it takes a couple of hours to run the cvs I've ended up with an
out of sync tree once that caused the build to fail at another point,
but I've hit the properties.h bug 3 times.

What is the recommended way to clean out the /usr/src tree just in case
there's cruft laying around that is causing the properties.h problem?  
I may try just deleting and regenerating the tree from scratch, but it
seems like there would be a better way to scrub the tree so I'm not cvs
co'ing the whole tree again.

Thanks,
Heitzso