Windows 7 support on Rust 1.77 and lower

[ensch]

Currently the Window 10 and newer code using ProcessPrng is always chosen on Rust 1.77 and lower, because target_vendor = "win7" was only introduced with Rust 1.78. As the ProcessPrng API does not exist on Windows 7, programs just refuse to start.

Code in backend.rs:

 } else if #[cfg(all(windows, target_vendor = "win7"))] {
        mod windows7;
        pub use windows7::*;
    } else if #[cfg(windows)] {
        mod windows;
        pub use windows::*;

Would it be possible to either always use the safe Windows 7 code on older Rust versions (1.77 and lower) or change the code to automatically detect if ProcessPrng is available using e.g. GetProcAddress, so the best API is chosen depending on what is available? All older Rust versions support Windows 7 by default, so it seems wrong to statically depend on a Windows 10 API with no way to disable this dependency.

[newpavlov]

Hm, I think we could add Rust compiler version detection to our build script and switch to the windows7 backend on older compilers. Alternatively, we could introduce a getrandom_backend = "windows_legacy" configuration flag.

[newpavlov]

I decided to try the build script approach. See the PR linked above for more information.