BUILD_FLAT_MESH needs to be enabled at build time so that the functionality is added to the MeshPart module that is used by the Mesh Design workbench.
This is not enabled for the PPA because of dependency challenges due to eign3, pybind11/libboost-python versions available.
So..., I don't foresee this being added to Ubuntu 16.04, I would suggest @erich use the AppImage. I know this isn't as convenient as using a native package manager, but 16.04 has become a nightmare to maintain with all the newest FreeCAD dependencies. I know @kkremitzki has worked hard to get the PPA working, but some compromises needed to be made (notice you are still using QT4). The .AppImage is using Conda as the package manager so the dependencies are advanced. @UR_ is using Conda based builds on Win, that's why it works for him.
We may be able to add a newer eigen3 to the PPA for 18.04, but I'm not sure. I will think of a way to test this with our Travis-CI support.