Refreshed patch, with a few minor updates:

 - Use copysign instead of signbit (we don't currently check for signbit
   in the configure scripts); only check for negative zero when we've
   already checked equality with 0.0.

 - Use Py_IS_NAN and Py_IS_INFINITY instead of isinf and isnan.

 - Add an extra check for really tiny numbers in _PyFloat_Pack2 to avoid binary64 subnormals in intermediate calculations.

 - Remove a duplicate check for the case x == 0.0 in _PyFloat_Pack2.

 - Make unpacking of ints and nans behave in the same way as float("inf") or float("nan"): use _Py_dg_infinity and _Py_dg_stdnan if available, else fall back to Py_HUGE_VAL and Py_NAN.

 - Minor style nits (braces; use floating-point constants when comparing with floats; consistent capitalization for hex literals, etc.)