Stage 4 of dynamically linked root support. Add a big knob,
WITH_DYNAMICROOT, which will toggle the generation of dynamically-linked
binaries for installation in /bin and /sbin. It is currently off,
meaning that /bin and /sbin are still statically linked by default.

If something goes wrong (which I hope doesn't), this is what /rescue is
all about. Please do not try to use WITH_DYNAMICROOT and NO_RESCUE to
save space or some other equally silly reason. If you do and end up
having problems, you have been warned.