[cmake][bugfix] Make CMake Doxygen creation work for modules without Doxylocal

When creating Doxygen documentation, CMake currently assumes that all modules have a Doxylocal file, which is in fact only present in dune-common right now. That file is then used as both a dependency and a file reading input, causing CMake errors.

This patch fixes the problem by testing for the presence of Doxylocal and only including it in the build process if it was found.