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                        MSYS2 Mingw-w64 cmake failed
following the README in the part of Building. cmake says "System is unknown, create Platform/MINGW_NT-10.0..." and "Checking for module 'gtk+-3.0'"

Very strange; I think the warning at the top of the output there has something to do with it (CMake is thinking this is Unix and not Windows).
Can you try with meson in the current head?
[This can probably be closed as the user account is no longer there. Hopefully meson fixed these issues.]
Just leaving some breadcrumbs in case someone else has this problem...
I think it depends on which version of cmake you have installed - the msys2 version, or the windows build from the cmake website. I forget the details, but I think they make different default assumptions about various things, like which compiler to use and how to scan for dependencies. Hopefully a moot point now the switch has been made to meson!