Add compilation flags to rosdistro

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From your description I have the impression this library is not a ROS package.

A release would then mean doing a Bloom 3rd party release, which supports adding ROS-release specific patches. I've linked to the first section about patches (adding an install rule + manifest), but the same mechanism can be used for arbitrary other patches.

Using a patch you should be able to change the CMakeLists.txt and update the default values for the options you refer to.

I'm not sure it's possible to update the actual command that gets used by the buildfarm to build the package, but changing the defaults for the CMake options would seem to be a good alternative.

Edit: just remembered there is actually direct support for changing the build command used: Changing the build flags for a package.

To release a non-ROS (actually: Catkin) package though, you'd still need to do a 3rd party release, so I've left the first part of my answer above.

To answer your specific questions:

Is it possible to set compilation definitions for the ros buildfarm? Somewhere in the rosdistro's kinetic/distribution.yaml file maybe?

It should be clear now that you don't configure those flags in distribution.yaml, but they are part of the build configuration stored in your release repository.

Is it possible to define different definitions for different platforms?

Yes. Just make sure to checkout the correct branch in your release repository if changing the flags directly or when setting up the patches for a release.