Fix several Windows build warnings (#59634)
Should fix:
- `[-Wstrict-prototypes]`
- `[-Wattributes]`
- `[-Wc++-compat]`
- `[-Wunused-but-set-variable]`
- `[-Wdiscarded-qualifiers]`
- `[-Wundef]`
This leaves:
- `[-Warray-bounds]`
- `[-Winfinite-recursion]`
see https://github.com/JuliaCI/julia-buildkite/pull/484
