maint: improve remake rules for maintainers

maint: improve remake rules for maintainers

This is a follow up on today's commit v8.19-60-g4f2e62b".

* Makefile.am ($(top_srcdir)/m4/cu-progs.m4,
$(srcdir)/src/cu-progs.mk): New, generate these files from the
'build-aux/gen-lists-of-programs.sh', the same way it's done
from the bootstrap script.
* bootstrap.conf (bootstrap_post_import_hook): Add comment about
the necessity to keep those new rules synced with the commands
here.  Enhance those commands so to that the generated files are
set read-only.