[build] Add a ChangeLog generation rule
[build] Add a ChangeLog generation rule

Just like Clutter, we need to generate a proper ChangeLog when
distchecking so that users of the tarball will still have a
detailed list of changes.

Distrotech/clutter-gtk,GNOME/clutter-gtk,Distrotech/clutter-gtk,GNOME/clutter-gtk,Distrotech/clutter-gtk