<doxygen> elements can influence Doxygen. <br/>
<doxygen> nested elements can influence Doxygen. <br/>
<doxygen> nested elements take precedence over <doxygen> non-nested elements. <br/>
the Ant task should whine, but not fail if the user claims ignoreVersion="true" and the detected Doxygen version is too old. <br/>
the Ant task should not fail if the detected Doxygen version is minimally supported. <br/>
The amalgamated configuration file basis can be controlled by the Ant task user.