Draft: Fix issue with make looking in LHCb externals for internal library used by Geant4

See details in !974 (comment 6945404), where this is picked up from

cc: @dpopov, @veltri, @witoldp, @clemenci

This is needed for Sim10 build against G4 10.7.

To check if Gauss-on-Gsino or Gsino needs it or it is already taken care of, cc @mimazure