Something

	<target name="clean-unless-needed" unless="${builddir.exists}">
		<echo message="now: ${builddir.exists}" level="info"/>
		<delete>
			<fileset dir="${dir.build}" includes="**/*.class"/>
		</delete>
		<!-- echo message="deleted contents of ${dir.build}" level="info"/ -->
	</target>

	<target name="clean-if-needed" if="${builddir.exists}">
		<echo message="now: ${builddir.exists}" level="info"/>
		<delete>
			<fileset dir="${dir.build}" includes="**/*.class"/>
		</delete>
		<!-- echo message="deleted contents of ${dir.build}" level="info"/ -->
	</target>