1. Fluid pharmaceutical composition which allows the controlled release of at least one active substance and which comprises a) a therapeutically effective amount of at least one active substance, b) from 3 to 55% by weight of phospholipid, c) from 16 to 72% by weight of pharmaceutically acceptable solvent, and d). from 4 to 52% by weight of fatty acid, said composition having the property of gelling instantaneously in the presence of an aqueous phase. 2. Pharmaceutical composition according to claim 1, characterized in that the active substance is chosen from antibiotics, anti-infectious agents, local anesthetics, anti-inflammatory agents, anti-mycotic agents and peptide active substances. 3. Pharmaceutical composition according to either of claims 1 and 2, characterized in that the phospholipid is chosen from phosphatidylcholine, phosphatidylglycerol salts, dicaproylphosphatidyl-choline and distearoylphosphatidylglycerol salts or a mixture thereof. 4. Pharmaceutical composition according to claim 3, characterized in that it contains from 15 to 55% by weight of phospholipid, preferably from 15 to 51% by weight of phospholipid. 5. Pharmaceutical composition according to either of claims 1 and 2, characterized in that the phospholipid is a hydrogenated phosphatidylcholine. 6. Composition according to claim 5, characterized in that it contains from 3 to 11%, preferably from 3 to 10%, by weight of phospholipid. 7. Composition according to any one of claims 1 to 6, characterized in that the pharmaceutically acceptable solvent is chosen from propylene giycol, polyethylene glycols and mineral oils such as liquid paraffin or silicone oils or a mixture thereof. 8. Composition according to any one of claims 1 to 7, characterized in that the fatty acids used are saturated or unsaturated organic carboxylic acids containing from 4 to 22 carbon atoms, preferably from 8 to 18 carbon atoms. 9. Pharmaceutical composition according to claim 8, characterized in that the fatty acids are chosen from oleic acid, caprylic acid, capric acid, caproic acid, myristic acid and butyric acid or a mixture thereof. 10. Pharmaceutical composition according to any one of claims 1 to 9, characterized in that further comprises up to 5% by weight of monoglyceride or of diglyceride or of a mixture of mono- and of diglyceride, and/or up to 15% by weight of triglycerides. 11. Method for manufacturing a pharmaceutical composition according to any one of claims 1 to 10, characterized in that it comprises the following successive steps: i) the phospholipid(s) is (are) dissolved in the pharmaceutically acceptable solvent; ii) the fatty acid(s) is (are) added to the phospholipid solution with stirring; iii) the active substance(s) is(are) incorporated into the mixture obtained at. the end of step ii), and iv) water is optionally added to the composition obtained in step iii). 12. Method according to claim 11, characterized in that the active substance(s) is (are) dissolved in a minimum amount of water before the incorporation in step iii). 13. Method according to claim 11, characterized in that the active substance(s) is (are) incorporated in step iii), optionally in micronized form. 14. Use of a composition according to any one of claims 1 to 13 for the controlled release of one or more active substances by subcutaneous and/or intramuscular injection of active substances.