auto-update Makefile

jom should support the feature known from gnu make to allow Makefiles to refresh themselves, following this algorithm:

start:
   read Makefile
   if there is a target matching the current Makefile:
      if it is outdated:
         execute commands
         goto start
   rest of makefile processing ...  

note that this is literally the algorithm: if the commands don't actually update the makefile, gmake goes into a loop. this is potentially useful, so i wouldn't bother limiting it.