NEURON is changing units values (CODATA 2018 constants), see : BlueBrain/mod2c#47
As discussed in BlueBrain/mod2c#47 (comment), we need to add -DCORENEURON_ENABLE_LEGACY_UNIT=ON option to CMake to enable/disable use of legacy units.

What needs to be done:

• add CMake switch CORENEURON_ENABLE_LEGACY_UNIT
• share/nrnunits.lib in MOD2C should become template file share/nrnunits.lib.in
• Depending on value of CORENEURON_ENABLE_LEGACY_UNIT option and using CMake's configure_file functionality, share/nrnunits.lib.in should be converted to share/nrnunits.lib at build time with old or new values of different constants.

The same mechanism was used in earlier NEURON versions (see usage of NRN_ENABLE_LEGACY_FR in cmake/ConfigFileSetting.cmake).

@iomaganaris can provide additional context.