Since Ant 1.7
The value of the ant.build.clonevm system property controls how Ant instruments forked Java Virtual Machines. The java and junit tasks support clonevm attributes to control the VMs on a task-by-task basis while the system property applies to all forked Java VMs.
If the value of the property is true, then all system properties of the forked Java Virtual Machine will be the same as those of the Java VM running Ant. In addition, if you set ant.build.clonevm to true and build.sysclasspath has not been set, the bootclasspath of forked Java VMs gets constructed as if build.sysclasspath had the value "last".
Note that this has to be a system property, so it cannot be specified on the Ant command line. Use the ANT_OPTS environment variable instead.