fix of OSX build: workaround of non-standard std::optional
fix of OSX build: workaround of non-standard std::optional

prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r