Duplicated urdfdom's method of including tinyxml

Third attempt to get TinyXML included correctly with CMake.

This is the exact method in URDFDom:
[SearchForTinyXML.cmake](https://bitbucket.org/osrf/urdfdom/src/3ccbac6bfbbe8c0f10998f872faf976ca5a9af50/cmake/SearchForTinyXML.cmake?at=default)

See:
https://github.com/ros-controls/ros_control/commit/8a55e3bb25bb50a91a7866e20d8af6d1077410de
https://github.com/ros/catkin/pull/486

@adolfo-rt @wjwwood