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• htole64() does not exist on MacOS, and leads to an error with a simple test like python -c "import pymargo" without the endian macro shim added here; other endian.h usages are also present and patched here

• SSG has the same problem with endian.h header macro usage, but one thing at a time..

• I verified this fix with a custom spack install mochi-margo@mybranch followed by clean re-install of py-margo and re-run above one-liner.