Macro warn_thread

macro_rules! warn_thread {
    { $fmt:expr $(,$arg:expr)* } => { ... };
}

Requires a pub static DO_WARN_THREAD: AtomicBool = AtomicBool::new(false); in the scope, which can be changed via ...::DO_WARN_THREAD.store(true, std::sync::atomic::Ordering::SeqCst);.