Change build system to auto-generate the ChangeLog
* build-aux/gitlog-to-changelog: New script. Taken from GnuPG. * build-aux/git-log-fix: New file. * build-aux/git-log-footer: New file. * doc/HACKING: Describe the ChangeLog policy * ChangeLog: New file. * Makefile.am (EXTRA_DIST): Add new files. (gen-ChangeLog): New. (dist-hook): Run gen-ChangeLog.
Rename existing ChangeLog files to ChangeLog-old.