• Option to disable TLS verification (discourage unless your computer has broken certificates, just for debugging) -DCMAKE_TLS_VERIFY=off. Default is TLS verify on
• Option to fall back to unmaintained Zlib 1.x. Zlib 2.x works on most systems and is significantly faster for compressed HDF5 operations. -Dzlib_legacy=on. Default is to use Zlib 2.x
• check that TLS working from CMake before downloading to avoid confusing errors when user's CMake doesn't have SSL or TLS certificates are broken.

gemini3d.run: correct behavior on Windows. Also in general allow running from same directory or other directory.