Builds on Windows have confusing warnings

[anacrolix]

Per #19074 (comment):

Git-Bash_full_log.txt This is the same error I get both in my Windows 11 VM and full installation. Would be nice to remove all warnings.

No warning at all if compiling wmake.ps1 in file explorer or Powershell (old method).

Builds on Windows with warnings confusingly look like the build failed. #17214 probably fixes the warning @bloxster is seeing. I think due to the build instructions using a compiler derived from GCC.

I wonder if we want a CI workflow that checks there's no warnings when building on Windows, or something else.