CMakeLists: automatically download and use wiringPi

Raspboop will automatically fetch the latest version of the wiringPi
library from Github (https://www.github.com/WiringPi/WiringPi),
extract the zip file, build wiringPi, and create a hard link to the
library with an appropriate naming convention for CMake.

Users do not need to have wiringPi on their system before building
Raspboop.