Update the release-message target to generate the full release
announcement out of the standard blurb and the NEWS file. We use
a pretty evil sed incantation, courtesy of Neil Roberts.
The check-news option in configure.ac conflicts with the idea of using a
buildbot to do a distcheck.
Since we're doing some validation on the state of the build during the
release-check phase we should add the NEWS file check there.
http://bugzilla.clutter-project.org/show_bug.cgi?id=2468
