Annoying cmake warning

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This looks like a bug in the CMakeLists.txt in the keyboard package.

The catkin_package macro is used to declare which files, libraries and packages need to be included in C++ when another package depends on this package. It looks like they're trying to export a dependency on SDL but instead of declaring that downstream packages should depend on a specific object file, it should just declare a depends on SDL.

Of course, because this package only exports messages, and those messages don't need SDL to compile, it doesn't need to export the dependency on SDL at all.

If you want this fixed I suggest you file a ticket on the project's bug tracker and link back to this answer.