Resolve "AM_SILENT_RULES issue with automake 1.17"

Closes #3524 (closed).

• 460ca77d workaround for silent rules being ignored in automake 1.17

  Also remove all the few other trailing dnls just to be safe.

• 62593ab8 unrelated, but missed in #3522 (closed): add changelog.dox to tarball

• 7c117454 add stamp-h* to gitignore

  Under some unknown conditions, you can get stamp-h2, and it probably keeps incrementing.

• https://jenkins.aws.isc.org/job/kea-manual/job/ut-extended/309/

• https://jenkins.aws.isc.org/job/kea-manual/job/ut-distcheck/15/