I didn’t find a fix. I migrated from 3.6.9 which did not use the .vst3 folder format for Windows. So I left it that way (which means the SMTG_CREATE_BUNDLE_FOR_WINDOWS is set to OFF so the first IF is never executed).
But yes that is clearly an issue that needs to be fixed. One way to address it is to copy the file VST3_SDK/cmake/modules/AddSMTGLibrary.cmake into your own cmake folder (ex: your_project/cmake), fix the problem and add
list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}/cmake")
before including anything from VST in your cmake file. I did this in Jamba (https://github.com/pongasoft/jamba/blob/v5.0.0/jamba.cmake#L28) to override a compile option set in PlatformToolset.cmake that was causing huge number of unecessary warnings (-Wsuggest-override)