CMake: fail pipeline and/or build when some sources are unused

In the current Lbcom/master:CMakeLists.txt is missing VP/VPMonitors. The code there can't even build at the moment (was obviously missed when modernizing).

This particular case is not so frequent but it can happen that a cpp is added which is not listed in the CMakeLists. We can add some checks for this somewhere, similarly to what's done in Gaudi

/cc @tmcgrath