portable file touched during boot

[Andrew Cagney]

I'm trying to hack up a Make rule that's only run once after a reboot
by having it depend on a file touched during boot.  Something like
   foo: /proc/uptime
         do stuff
         touch foo
On Linux, /proc/uptime seems promising - it has the boot time (other
files such as /proc/1 seem to have 'localtime') -
however, on NetBSD /proc/uptime has the current time :-/
Any suggestions.