Makefiles.
DANGER WILL ROBINSON!
This will cause repeat installs of certain programs, such as `init' and
`rcp', to fail unless one of the two conditions is met:
1) You are in single-user mode.
2) Your security level is set to 0 or -1.
If you have compiled a kernel from the latest sources, your kernel
security level is set to -1 by default, which will keep `init' from
fiddling with it. You can increase it, but not decrease it, from the
command line with the command `sysctl -w kern.securelevel=<new value>'.
I believe that -1 is the most appropriate value to use while we are still
developing the code, although when we ship it should be changed back to 0.
See init(8) for more information.