Run make regen automatically.

[schwern]

It's bad enough that one cannot simply write a C function, you have to add it to embed.fnc, but at least we can make regenerating the headers automatic.

The original rationale for not doing so was because make regen relies on Perl being installed and that can lead you to a weird state if Perl is not already installed or it's broken or it's old. Back in 1997 that could happen. Here in 2015 it's pretty unlikely.

It also comes from the POV that only a few people will be working on the Perl core and only occasionally writing new functions. We encourage lots of people to work on the Perl core and to write lots of new, small functions.

This will probably involve adding targets to the Makefile for each generated header file so that it runs its appropriate script in regen/ .. What is built by each script can be found by grep -r 'This file is built by .*blah.pl' .. The dependencies will require reading through each regen script. Usually you can just look for the open statements. For example...

opcode.h opnames.h lib/B/Op_private.pm pp_proto.h: regen/opcodes 
    -perl regen/opcode.pl