Compiles against official choreonoid and export package

[arntanguy]

This PR is related to https://github.com/choreonoid/choreonoid/pull/5:

• It ensures that choreonoid_ros links against the official choreonoid master (>= 1.8.1)
• It exports the (empty) choreonoid_ros package such that downstream packages can know whether choreonoid has ros support

  find_package(choreonoid_ros QUIET)
  if(${choreonoid_ros_FOUND})
  # do specific things if choreonoid has ros (install ROS-specific project files, etc)
  endif()