When using CMake 3.19 or newer and no CMAKE_CXX_STANDARD has been set before the call to find_package(dd4hep), downstream code will error with:
    ```
    CMake Error at /afs/cern.ch/user/v/vavolkl/cmaketest/tmp/CMakeFiles/CMakeTmp/CMakeLists.txt:13 (add_executable):
      CXX_STANDARD is set to invalid value 'CXX_EXTENSIONS'

    CMake Error at CMakeLists.txt:3 (try_compile):
      Failed to generate test project build system.
    ```
    because ${CMAKE_CXX_STANDARD} is empty. I couldn't find any relevant information in the changelog, so this might be a bug in CMake proper. When configuring dd4hep, CMAKE_CXX_STANDARD is assumed to always be set.