No longer maintain a version-controlled ChangeLog file, but do
continue to include a ChangeLog file in distribution tarball.
* Makefile.am (gen-ChangeLog): New rule.
(dist-hook): Depend on it.
(EXTRA_DIST): Add ChangeLog-old.
* bootstrap (modules): Add gitlog-to-changelog.
* ChangeLog: Remove file.  Renamed to...
* ChangeLog-old: ...this.  New file.
* autogen.sh: Touch ChangeLog, to ensure it exists.  For automake.