Sets a property if a resource is available at runtime. This resource can be a file, a directory, a class in the classpath, or a JVM system resource.
If the resource is present, the property value is set to true by default; otherwise, the property is not set. You can set the value to something other than the default by specifying the value attribute.
Normally, this task is used to set properties that are useful to avoid target execution depending on system parameters.