Merge branch 'devel-2.3'

* devel-2.3:
  devel/release: Use release-specific Makefile targets
  Makefile: Add new dist target for releases
  Makefile: Stricter checks for release distchecks

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>