Fix OpenMP issue on macosx and implement proper dependencies

OpenMP comes with the proper targets which are used now.

Implement proper dependencies

With the change to the CMake targets the dependency tree was broken. The generation of the ROOT dictionary is now a target on its own which did not depend on anything except the header files needed for the specific library. Some of these header files include other header files from the external libraries. The problem was that now it became possible that the dictionary generation started before the external header files were available.

Add explicit dependencies from these dictionaries to the external libraries.