Here they say, it should not be used anymore. It is already deleted from the GitHub-repository. However you can still download it from their site as they have not (yet) deleted it from there. Then, the files in
VST2_SDK should be copied to the folder with the VST3 SDK. It should be possible to copy over those files to the location where you saved this repo under and get it to compile.
EDIT: Well, it should work,... but it is such a mess that I have tried almost any combination of paths already. So I guess I am also stuck here. I saw Rory did already say to put some work into the Linux build, so we have to wait a bit.
Also, the move from Steinberg to just delete it without any warning beforehand just sucks. They did even have things like two different versions of the same version number in the past.