Handy macro for complex conditional compilation

if_cfg.rs

// Feel free to copy and modify this code if it's useful to you. 
macro_rules! if_cfg {
    ($(#[cfg($cfg:meta)] $it:item)+ fallback: $els:item) => (
        $(#[cfg($cfg)] $it)+
        #[cfg(not(any($($cfg),*)))] $els
    )
}

if_cfg {
    #[cfg(target_arch="x86")]
    fn arch() -> &'static str { "x86" }
    #[cfg(target_arch="arm")]
    fn arch() -> &'static str { "arm" }
    fallback:
    fn arch() -> &'static str { "other" }
}