[DRE-maint] Packaging setup.rb?

[Vincent Fourmond]

> This doesn't work in the case that the upstream source has an old
> setup.rb.

  It should be painless to check that in the makefile, save the old
version, and restore it on make clean. That could (should ?) be
incorporated into the ruby-setup-rb.mk class, making it
straightforward. Packagers could opt to keep the old version of
setup.rb if found, in case of backward compatibility problems.

  Cheers !

      Vincent