/etc/rc.d/rc.sysinit regenerates /boot/kernel.h every time the machine
reboots.  This means that every time you run "gmake" in a kernel tree after
a reboot, lots of files will be rebuilt because they depend on
/boot/kernel.h (which is included by version.h), even though its content
hasn't really changed.

rc.sysinit should generate kernel.h in a temporary file, then compare it to
the one that's already there; if it's different, it should replace the old
one, but if it's the same, the temporary file should be removed and the old
kernel.h should be left in place.