Can be downloaded here. Included in the link are instructions for various platforms. Make sure in the end of installation the development libraries are available on your build path.
This is how one would do it in Ubuntu Linux 1416.04
Enter the following commands: sudo apt-get update sudo apt-get install build-essential libssl-dev libc-ares-dev uuid-dev mkdir ~/dev cd ~/dev wget http://mosquitto.org/files/source/mosquitto-1.4.810.tar.gz tar zxvf mosquitto-1.4.810.tar.gz cd mosquitto-1.4.810 make sudo make install