1. Solid pharmaceutical composition for the controlled release of perindopril or a pharmaceutically acceptable salt thereof, characterised in that it comprises a thermoformable mixture of perindopril or a pharmaceutically acceptable salt thereof and of one or more polymers selected from the group of the polymethacrylates, the release of the perindopril being controlled solely by the nature of the polymethacrylate(s) used, by the amount thereof relative to the perindopril and by the technique employed in the manufacture of the said composition. 2. Solid controlled-release pharmaceutical composition according to claim 1, characterised in that the polymethacrylate(s) used in the thermoformable mixture belong(s) to the family of the Eudragit(RTM) products RL and/or RS. 3. Solid controlled-release pharmaceutical composition according to either claim 1 or claim 2, characterised in that the polymethacrylate(s) used in the thermoformable mixture is/are Eudragit(RTM) RLPO and/or Eudragit(RTM) RSPO. 4. Solid controlled-release pharmaceutical composition according to any one of claims 1 to 3, characterised in that the thermoformable mixture comprises Eudragit(RTM) of type E, alone or in association with one or more of the Eudragit(RTM) products mentioned hereinbefore. 5. Solid controlled-release pharmaceutical composition according to any one of claims 1 to 4, characterised in that the thermoformable mixture comprises Eudragit(RTM) of type L100, L100-55 and/or S100, alone or in association with one or more of the Eudragit(RTM) products mentioned hereinbefore. 6. Solid controlled-release pharmaceutical composition according to claim 1, characterised in that the said composition is administrable by one of the routes selected from the oral, buccal, sublingual, ocular, vaginal, rectal and parenteral routes. 7. Solid controlled-release pharmaceutical composition according to any one of claims 1 to 6, characterised in that the said composition is administrable by the oral route. 8. Solid controlled-release pharmaceutical composition according to claim 1, characterised in that the temperature of thermoforming of the mixture is from 60 degree C to 150 degree C. 9. Solid controlled-release pharmaceutical composition according to either claim 1 or claim 8, characterised in that the temperature of thermoforming of the mixture is from 80 degree C to 130 degree C. 10. Solid controlled-release pharmaceutical composition according to claim 1, characterised in that the mixture is thermoformed according to the technique of extrusion. 11. Solid controlled-release pharmaceutical composition according to claim 1, characterised in that the mixture is thermoformed according to the technique of injection. 12. Solid controlled-release pharmaceutical composition according to claim 1, characterised in that the mixture is thermoformed according to the technique of co-extrusion, the inner layer of the said composition in this case being composed of the said mixture and the outer layer of the said composition being composed either of one or more polymethacrylate(s) or of one or more polymethacrylate(s) in admixture with perindopril or a pharmaceutically acceptable salt thereof. 13. Solid controlled-release pharmaceutical composition according to claim 1, characterised in that the mixture is thermoformed according to the technique of co-injection, the central portion of the said composition in this case being composed of the said mixture and the outer layer of the said composition being composed either of one or more polymethacrylate(s) or of one or more polymethacrylate(s) in admixture with perindopril or a pharmaceutically acceptable salt thereof. 14. Solid controlled-release pharmaceutical composition according to claim 1, characterised in that it optionally contains one or more pharmacologically acceptable excipients selected from anti-oxidants, flavourings, colourings, preservatives, sweeteners and anti-adherents. 15. Solid pharmaceutical composition according to claim 1, characterised in that the perindopril is in the form of the tert-butylamine salt.