Compilation with LCG_97 fails

To make use of some bugfixes and new features in ROOT/v6.20, I want to compile Urania using LCG_97. LCG_97 has just been deployed at /cvmfs/lhcb.cern.ch/ and seems to be picked up correctly when running

HEPTOOLS_VERSION=97 make

After that there are several CMake warnings about a "dependency on unknown subdirectory", e.g.

CMake Warning at cmake/GaudiProjectConfig.cmake:1407 (message):
  Subdirectory 'Phys/Ks2MuMu' declares dependency on unknown subdirectory
  'Math/SomeUtils'
Call Stack (most recent call first):
  cmake/GaudiProjectConfig.cmake:589 (gaudi_collect_subdir_deps)
  CMakeLists.txt:12 (gaudi_project)

But CMake seems to be fine and compilation starts, but fails with errors of the kind

g++: error: unrecognized command line option ‘-std=c++modules’

I cannot see where this command line argument is coming from. My attempt to grep -r for it was not successful.