The install target of cn-cbor's Makefile fails when the documentation
files have not been generated. To fix this, the doc target is
explicitly invoked before install.
.travis.yml
Add in the gnutls build.
build-env/Dockerfile.build-env
build-env/Dockerfile.develop
Add in libgnutls28-dev to the apt-get list.
scripts/build.sh
scripts/dist.sh
Support TLS set to gnutls for building.