Install docker. For OSX, I recommend using dvm to get virtualbox, vagrant and boot2docker set up correctly.
Build:
$ cd support
$ docker build -t your-username/stunnel-builder .
$ docker run -i -v /home/docker/cache:/var/cache \
-e STUNNEL_VERSION=<stunnel-version> \
-e AWS_ACCESS_KEY_ID=<ur-key> \
-e AWS_SECRET_ACCESS_KEY=<ur-secret-key> \
-e S3_BUCKET="<ur-bucket-name>" \
your-username/stunnel-builder
heroku plugins:install https://github.com/heroku/heroku-buildpacks
cd heroku-buildpack-stunnel
git checkout master
heroku buildpacks:publish <ur-user-name>/stunnel