message: Make example cross platform
  Closes
diff: @@ -21,22 +21,29 @@ winapi = "0.3"
  ```
  main.rs:
  ```Rust
  -extern crate winapi;
  -use std::ffi::OsStr;
  +#[cfg(windows)] extern crate winapi;
  use std::io::Error;
  +
  +#[cfg(windows)]
  +fn print_message(msg: &str) -> Result<i32, Error> {
  + use std::ffi::OsStr;
  use std::iter::once;
  use std::os::windows::ffi::OsStrExt;
  use std::ptr::null_mut;
  use winapi::um::winuser::{MB_OK, MessageBoxW};
  -
  -fn main() {
  - let msg = "Hello, world!";
  let wide: Vec<u16> = OsStr::new(msg).encode_wide().chain(once(0)).collect();
  let ret = unsafe {
  MessageBoxW(null_mut(), wide.as_ptr(), wide.as_ptr(), MB_OK)
  };
  - if ret == 0 {
  - println!("Failed: {:?}", Error::last_os_error());
  + if ret == 0 { Err(Error::last_os_error()) }
  + else { Ok(ret) }
  +}
  +#[cfg(not(windows))]
  +fn print_message(msg: &str) -> Result<(), Error> {
  + println!("{}", msg);
  + Ok(())
  }
  +fn main() {
  + print_message("Hello, world!").unwrap();
  }
  ```
  