update-copyright local hook

[Joel E. Denny]

For Bison, update-copyright is not enough.  Our backend skeletons contain 
m4 macro invocations with lists of copyright years as arguments.  I've a 
written a script and a makefile target to handle these, but I would like 
it to be called by maint.mk's update-copyright target rule so we don't 
forget it.  What's the best way to implement an extension like this?