maven-antrun-plugin and ant-optional

[exquisitus]

I tryed to run some ant tasks from within ant. Unfortunatly the
execution fails if I have to use a optional ant-task.
 
My POM looks like this:
 
                  <plugin>
                        <artifactId>maven-antrun-plugin</artifactId>
                        <executions>
                             <!-- generate the XSD documentation -->
                             <execution>
                                   <phase>validate</phase>
                                   <configuration>
                                         <tasks>
                                               <ant antfile="build.xml"
target="gen-xsddoc">
                                                     <property
name="doctitle" value="${pom.name} - ${pom.version}" />
                                               </ant>
                                         </tasks>
                                   </configuration>
                                   <goals>
                                         <goal>run</goal>
                                   </goals>
                             </execution>
                        </executions>
                        <dependencies>
                             <dependency>
                                   <groupId>ant</groupId>
                                   <artifactId>ant-optional</artifactId>
                                   <version>1.5.3-1</version>
                             </dependency>
                        </dependencies>
                  </plugin>
 
 
has anyone an idea how to enable the optional tasks for ant?
 
Regards domi