Import GaudiGSL from Gaudi

To be able to license Gaudi as Apache 2 it was decided to remove GaudiGSL (GPLv3) from Gaudi and import it in LHCb.

This is the first part of the move. It's meant to be backward compatible with versions of Gaudi still including GaudiGSL.

Note: this is needed once gaudi/Gaudi!879 (merged) is merged.