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

This is needed for Sim11 build against G4 10.7 in order to be able to run directly from the nightly install area for LHCbPR and later from release area in production.

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

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