useorocos: use build dependencies as provided by catkin_package_xml()

Instead of parsing the `package.xml` manifest with xpath we can simply piggyback the `${PROJECT_NAME}_BUILD_DEPENDS` variable that is already set from the `catkin_package_xml()` call for catkin builds.

References:
- https://github.com/ros/catkin/blob/indigo-devel/cmake/catkin_package_xml.cmake
- https://github.com/ros/catkin/blob/indigo-devel/cmake/parse_package_xml.py