[RFC] bpo-34585: Don't do runtime test to get float byte order

[rossburton]

Currently configure.ac uses AC_RUN_IFELSE to determine the byte order of doubles, but this silently fails under cross compilation and Python doesn't do floats properly.

Instead, steal a macro from autoconf-archive which compiles code using magic doubles (which encode to ASCII) and grep for the representation in the binary.

RFC because this doesn't yet handle the weird ancient ARMv4 OABI 'mixed-endian' encoding properly. This encoding is ancient and I don't believe the union of "Python 3.8 users" and "OABI users" has anything in. Should the support for this just be dropped too? Alternatively, someone will need to find an OABI toolchain to verify the encoding of the magic double.

https://bugs.python.org/issue34585

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[yan12125]

That's a great idea and it should help Android builds, too! A minor issue is - I remember in CPython, changes to ./configure should be added to the commit if configure.ac is modified.

[benjaminp]

This seems fine, but we should get a CLA.

[rossburton]

CLA sorted.

[benjaminp]

Thanks for the PR. I agree we should probably look into not pretending to support ARM4.