Support non-static build on windows

Currently, no export macros are defined, so we cannot build a DLL on Windows. (This also means that on other platforms, more symbols than necessary are exported, as we cannot set CXX_VISIBILITY_PRESET hidden in cmake).

I'd recommend using cmake's generate_export_header to accomplish this, cf. https://www.graphics.rwth-aachen.de:9000/OpenVolumeMesh/OpenVolumeMesh/blob/cmake-overhaul-2/src/CMakeLists.txt#L95