Bug 43362

This patch allows conditional attributes to be
used to suppress evaluation of unknown elements.
This has the effect of adding conditional execution for
tasks and conditon inclusion of nested elements.

Example usage:
<project name="x" xmlns:if="ant:if" xmlns:unless="ant:unless">

  <property name="onWindows" value="true"/>

  <target name="abc">
       <echo if:set="onWindows">I am on windows</echo>
       <echo unless:set="onWindows">I am *not* on windows</echo>
  </target>
  <macrodef name="mycommand">
     <attribute name="level" default=""/>
     <attribute name="debug" default=""/>
      <attribute name="file"/>
    <sequential>
       <exec executable="mycommand">
       <arg value="--debug"  if:true="@{debug}"/>
       <arg value="--level"    unless:blank="@{level}"/>
       <arg value="@{level}"  unless:blank="@{level}"/>
       <arg value="@{file}"/>
  </sequential>
 </macrodef>

</project>