build.sh trying to write to source tree

I'm unsure whether I'm doing something stupidly wrong.

Since my last cvs up, building -10 on -10 fails because nbhost-mkdep tries 
to write what.d.tmp into /usr/src-10/tools/what, which isn't writable for 
the user running the build.

Do I have some weird error in my build.sh invocation or is this a (known) 
regression?