For the record this should be fixed on meson's end (will be in upcoming >=1.3.0rc3) to emit a deprecation warning instead, see https://github.com/mesonbuild/meson/pull/12508
Not that the warning shouldn't be fixed here either way to avoid future issues.
The suggested fix to use vs_module_def breaks the build since MinGW targets ignore that apparently. We cannot fix this as-is.
I was a bit afraid of that but I didn't have the toolchain to build this locally.
It's a confusing situation since it was silently allowed for source files but not permitted for custom_target outputs... We really should be consistent and allow both. Well actually we should fix vs_module_defs to work correctly.
(I wish I'd known about this mingw failure mode a long time ago.)
Nothing extracted yet.
Usign meson-git (1.3.0rc1.r13.2d538c58c), the build now fails early on with: