That’s very odd! Are you using the latest JUCE version? The plug-in host with VST3 support is built automatically for Windows (with VS2017) with our build server every time we commit something, so it’s odd that we haven’t spotted this yet.
Using JUCE v5.1.2 build 18 Sep 2017. I got a different build error message with an older version of Steinberg. When I got the latest Steinberg, that error went away, but the new error popped up. I’m simply navigating to the plugin host folder and double clicking on the jucer file and building.
Our latest code definitely includes the flock.cpp file that contains those functions, so if you’re building with it, there’s not really any way it could be a missing symbol. I can only think that maybe you’re not building with the right paths or versions of juce/VSTSDK that you think you are.