Changes to WebAssembly targets and handling undefined symbols

// This program demonstrates that extern blocks must be marked unsafe
// in Rust 1.98.1 (edition 2024). The change to WebAssembly targets
// removes the --allow-undefined flag, so any extern function that
// is not linked will cause a compile‑time error unless it is
// explicitly marked unsafe. The following code compiles and
// prints a message, showing the correct usage.

fn main() {
    // Define an external C function. The block must be unsafe.
    unsafe extern "C" {
        fn js_log(n: u32);
    }

    // We don't actually call js_log here, but the compiler
    // accepts the definition because it is marked unsafe.
    println!("Extern function declared safely.");
}