Plugin should check for availability of source folder before attempting to run

The plugin should check for the existance of the maven.docbook.src directory before attempting the transformation. This will allow the property to be specified at a high level and the plugin will only execute if the lower level projects have the directory present. Here is a quick hack, perhaps there is a better way:

<ant:available file="$

" type="dir" property="maven.docbook.src.available"/>
<j:if test="$

">

....the rest of the script....

</j:if>