Support building on ZorinOS with SDL2 from apt

[JosiahWI]

Problem

My current setup for Minetest development is on ZorinOS. It provides development files for SDL 2.0.10 as libsdl2-dev from the Ubuntu repositories, but that package does not include SDL2's CMake target exports, so it can't be used to build your Minetest fork.

Solutions

I'd like to add support to the build for SDL2 packages with no CMake target exports.

Alternatives

We could have CMake fetch SDL2 automatically from git and configure it, or we could add SDL2 as a submodule. I don't like these solutions because SDL2 is a large download, and the shared system library runs perfectly fine in projects that support it.