[gecode] fix build against gecode 4.2.0 (closes #167329)
memory statistics have been removed from the search engines.
TODO list for the rql package
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* retirer les restrictions e_type = [A-Z][a-z_]*
r_type = [a-z_]*
* detecter les erreurs ASAP pour éviter stcheck
* tests unitaires pour sqlgen
* revoir tutoriel
* specification 1.0