OpenMP support is now switchable within the generation script

Since MSVC only has beta support for OpenMP 2.0 firstprivate cannot be used for these compilers. However, firstprivate is necessary for a reasonably safe implementation of OpenMP in MESA-PD. The solution is to move OpenMP support to the generator and disable OpenMP for the checked in version. The waLBerla OpenMP guards are removed as they are no longer needed. It is now assumed that if MESA-PD is generated with OpenMP support it is about to be used. If OpenMP is disabled with CMake warnings will pop up about undefined pragmas.