Make Cubos usable in CMake through find_package

[RiscadoA]

Problem

Currently our project can only be used by adding it as a subdirectory.
This isn't practical at all, and it also means it can't be installed system-wide - if you have multiple projects, you end up having multiple clones and builds of Cubos for each of them.
It also isn't standard - CMake libraries should export their targets.

Solution

We should investigate further on what we need to change to make this possible.