Add targetVersion support to Ant task

[uschindler]

The Ant task currently does not have support (like Maven or Gradle) to give the targetVersion as property to the task.

We should add this to allow a shorter notation:

<forbiddenapis failOnUnsupportedJava="false" targetVersion="${jdk.version}">
  <fileset dir="build/main"/>
  <signatures>
    <bundled name="jdk-unsafe"/>
    <bundled name="jdk-deprecated"/>
    <bundled name="jdk-non-portable"/>
    <bundled name="jdk-system-out"/>
    <bundled name="jdk-reflection"/>
  </signatures>
</forbiddenapis>