OpenMP on Windows

On Linux and macOS packages are built to allow OpenMP to be selected at runtime (see [docs]( https://conda-forge.org/docs/maintainer/knowledge_base.html#openmp )). Though it doesn't appear we have considered Windows. Admittedly am not a Windows expert, but my guess is this is due to Windows including its own (albeit somewhat dated) OpenMP implementation. As some folks are now investigating using LLVM OpenMP on Windows and Microsoft is looking at [improving their OpenMP offering]( https://devblogs.microsoft.com/cppblog/improved-openmp-support-for-cpp-in-visual-studio/ ), it may be worth discussing how we handle OpenMP on Windows and how we integrate that into how OpenMP is handled in conda-forge more generally.