[doxygen] Add target only if doxygen was found
The dune macro used to add the target only adds it if doxygen was found. Then the target was not know in add_custom_command which is an error in CMake > v3.2.
The dune macro used to add the target only adds it if doxygen was found. Then the target was not know in add_custom_command which is an error in CMake > v3.2.