[cmake] Fix dependency tracking for multiple operators

CMakes add_custom_command occasionally (undeterministically) trips over when multiple files are defined as output. The result is unnecessary regeneration upon every change of the accompanying C++ source...

The commit fixes the issue by defining one custom command per operator. This has the additional pro of generating more verbose messages during generation. Implementation wise, CMake needed to extract more information from the ini file, so I switched to using dune-testtools Python-CMake interface.

This fixes #108 (closed).