I don't think that the warning and the error are related. The warning is about a CMake policy that is not set. See the [documentation](
https://cmake.org/cmake/help/v3.0/policy/CMP0046.html) for an explanation. This is only a warning. To write your first message, you can follow [this](
http://wiki.ros.org/ROS/Tutorials/WritingPublisherSubscriber%28c%2B%2B%29) tutorial. Take a close look at the CMakeLists.txt part of the tutorial.
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