Require gitlog-to-changelog to be installed.
* Makefile.am (GITLOG_TO_CHANGELOG): New. (gen-ChangeLog): Use installed version of gitlog-to-changelog.
In case an appropriate version of gitlog-to-changelog is not in the
PATH, it is possible to override it using something like:
make distcheck GITLOG_TO_CHANGELOG=/foo/bar/my-gitlog-to-changelog