How to use conditional includes in xacro?

Hi all! When I'm using if-conditions in xacro like the example below, the substitution macro ($(find.... )) of the xacro:include statement will be evaluated independent of the if-result causing errors when the package is missing. I intend to have optional packages included which are not available by default. For this purpose I expected to use xacro:if analogously as the if-statement in launch files to enable or disable parts of the xacro file. Is there a proper way to model such things? (I'm using ROS Indigo with Ubuntu 14.04.) Thanks!