If I understood it correctly, AUv3 is for iOS and macOS, but AU is a macOS only format. So you shouldn’t compile juce_audio_plugin_client_AU_2.mm on iOS.
I’m also working on building JUCE projects using CMake in an open-source project called FRUT. There is no iOS support yet, but I’m actively working on it (I should be able to add the support by the end of the week, or next week). I’m always interested in having a look at how other people use CMake to build JUCE projects, so feel free to share something.
I beg to differ.
There are no reason to have some custom file to be opt-in or opt-out in the module depending on the platform.
That’s the rationale behind the juce module files where you should build everything.