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David, please could you also tell me whether HAVE_COPYSIGN is defined for those builds of Python? It should be in pyconfig.h in the top level of the build directory, if it is.
And (just to double check), at configure time, there should be a line in the output of the configure script that looks like
checking for copysign... yes
Do you get 'yes' or 'no' there?
If I had to guess, I'd say that it's the float -> hex conversion that's going wrong (so that (-0.0).hex() produces '0x0.0p+0' instead of '-0x0.0p+0'), and that this is caused by either a buggy system copysign function, or by the system copysign function not being found and Python using a buggy workaround.