this one just bit me again and I'm not sure if its something I did or if
its actually a bug. 

Doing a 'make build' with UPDATE=0 MKOBJDIRS=yes to get a full build I find
that for some circumstance libc builds nsparse.h in /usr/src/lib/libc
rather than in /usr/src/lib/libc/obj. When this happens libc doesn't build
and all sorts of things start breaking later. If I stop the build mv
nsparse.h into obj things work fine. 

Also if you've ever done a make obj and then the BSDOBJDIR target changes
the obj symlinks are all broken, but make obj doesn't fix this. Is this a bug?

--Chuck