We do require C++20 still, because designated initializers is part of
that standard. This is almost a revert, but conditionally selecting
between c++latest or c++20 when available, as that's what we really want.
Fixes: 55ca1c8db3 ("vulkan/microsoft: Remove `override_options: ['cpp_std=c++latest']` option for visual studio")
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15706>
f607db2689