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Ensure that the code paths for HAVE_UNWIND and HAVE_DBGHELP are never taken at the same time, even if the build file code changes. Prefer DbgHelp over libunwind on Windows in case both are somehow available because DbgHelp is only available when building with the MSVC toolchain, and libunwind won't give us debug symbols from objects built with the MSVC toolchain. Also, print slightly more useful messages for the level of stack trace support enabled, and document what each if conditional does.
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