maint: don't let envvar setting break ChangeLog generation

* Makefile.am (gen-ChangeLog): Clear amend_git_log when we
don't set it, so that an envvar setting cannot cause trouble.
maint: add gnulib's update-copyright module

* bootstrap.conf (gnulib_modules): Add update-copyright, so
that each year we can run "make update-copyright".
Otherwise, that rule fails due to the absence of the file
in build-aux.