Hi @fkostic, the build.cs file is specific to the Unreal engine and you can read more about it @ https://docs.unrealengine.com/en-US/Programming/BuildTools/UnrealBuildTool/index.html
This also might help you @ http://dmitry-yanovsky.com/2015/08/unreal-engine-4-build-file-demystified/
The original problem you are having is related to Unreal configuration not the Tobii API and so in order to use our Plugin with Unity, we do assume some level of familiarity with C++ and the Unreal.
Please go through the links above that should hopefully explain in detail how to proceed.