How to use DFly sources via nfs?

[walt]

I now have two DFly machines and I'm trying to spare DFly some bandwidth
by sharing the sources directory via nfs.
The problem is that quickworld halts immediately with missing file
errors, obviously related to the /usr/src symlink.
I have /usr from the first machine nfs_mounted (ro) on /mnt on the 
second machine.

I have /usr/src symlinked to /mnt/usr/src.

Is there an environment variable I need to set in order to make this
work?  Any other requirements?
Thanks!