Download the current Source packages from http://repo.mosquitto.org/debian/pool/main/m/mosquitto/. The Needed files are mosquitto_1.4.1.orig.tar.gz, mosquitto_1.4.1-0mosquitto1.dsc and mosquitto_1.4.1-0mosquitto1.debian.tar.gz

Put them in a base Directory like src/ and unpack the source using dpkg-source.

$ cd src
$ wget ...
$ dpkg-source -x mosquitto_1.4.1-0mosquitto1.dsc 
$ dpkg-buildpackage

If the build fails in make test, make sure that localhost ist resolvable and a user mosquitto exists..

sudo useradd -r -m -d /var/lib/mosquitto -s /usr/sbin/nologin -g nogroup mosquitto