Building android project(s) with ant for a newer sdk not working (configuration issue)

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http://stackoverflow.com – I have a large and old workspace with linked projects. But these projects were implemented for targetSdkVersion=8 and now I need to increase to targetSdkVersion=15. The problem occurs when I change to targetSdkVersion=15 in baseManifest.xml and try building with ant, the project is still built for targetSdkVersion=8. The build uses SDK files from a local project folder like {project-dir}/thirdparty/android/platforms/ and there are only android-4, android-7 and android-8 platforms in that folder. When I copy android-15 to that folder, update project and rebuild, nothing changes. There is no a (HowTos)