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split of issue3871 

There are some spelling errors in the comments. It would also be good to keep the indentation, quoting, layout etc similar to the surrounding code.
Regarding the comments, what is the significance of "runtime platform" vs MACHDEP? Also, what is the significance of sys.platform() when setting _PYTHON_HOST_PLATFORM?
Why did you use different types of comments? (# vs dnl)
I do not understand the comment "build must not depend from posix-compatible environment". Is this just explaining that we are setting ac_sys_system (uname -s) to a dummy generic value that will not trigger any special cases. Should we also set ac_sys_release to an empty string, like the cross-compiling case later on?
What’s the go with the two competing "mingw*)" cases when determining _PYTHON_HOST_PLATFORM? What is the situation where $host matches *-*-mingw*, but $host_os does not match mingw*?

Here is a modified patch fixing a couple of the spelling & style problems. I moved the comment introducing the ac_sys_ variables up to before the code that sets them. I rearranged the logic for _PYTHON_HOST_PLATFORM so it is clear that $host_os overrides $host, and I combined the cygwin and mingw cases.
I suspect the configure.ac logic could be simplified more.