Add a ChangeLog generator
Add a ChangeLog generator

We missed to add this feature when porting OpenSCAP to CMake.
This target serves to generate a ChangeLog from Git history.
The Changelog can be included in the source tarball in future.
Users can generate the ChangeLog by issuing `make changelog`
in the build directory.

OpenSCAP/openscap,redhatrises/openscap,OpenSCAP/openscap,jan-cerny/openscap,jan-cerny/openscap,OpenSCAP/openscap,jan-cerny/openscap,redhatrises/openscap,OpenSCAP/openscap,redhatrises/openscap,jan-cerny/openscap,redhatrises/openscap,mpreisler/openscap,redhatrises/openscap,OpenSCAP/openscap,jan-cerny/openscap,mpreisler/openscap,mpreisler/openscap,mpreisler/openscap,OpenSCAP/openscap,jan-cerny/openscap,mpreisler/openscap,mpreisler/openscap,redhatrises/openscap