Rationale: I have a project.json file, but I'm also generating C3 source files from CMake, which I would like to be included in the build. However they should not be generated next to the normal source files, but in the build tree, which could by anywhere.

So I propose a feature to allow adding extra source files for the build commands, which would effectively be additional entries in the "sources" array in project.json added from the command line.