* add support for cups dependencies
libcups has its own cups-config tool rather than using pkg-config.
This adds support for cups-config, based on pcap-config and
sdl2-config implementations.
This change also includes the unit test case and documentation for
cups dependency object implementation, and libcups2 dep to CI image.
This allows us to more easily have the documentation in sync with
the source code as people will have to document new features etc
right at the time where they implement it.
Iñigo Martínez