Its a feature! Not,its more of a bug. Ant 1.8 change how the javac task includes the ant runtime libs. The work around is to rather than set includeantruntime to false in the javac task set a property named, build.sysclasspath, to the value, last. why?
This way your builds will not break when changing ant versions and you are still offered the option to set includeantruntime to true when you really actually need it.