add_custom_command fails when OUTPUT is generated into a subdirectory

Excellent. I will go through my project, because until now I always added the header files as PUBLIC. I will switch that to PRIVATE or completely remove the keyword. I guess default would be PRIVATE, right?! I am going to find that out…

I will probably still add the headers (as PRIVATE), since my colleagues are using other IDEs, e.g. Visual Studio and Eclipse CDT.

Thanks again for the information!