Macro f

macro_rules! f {
    ($(
                $(#[$attr:meta])*
                pub $({$constness:ident})* fn $i:ident($($arg:ident: $argty:ty),* $(,)*) -> $ret:ty
                    $body:block
            )*) => { ... };
}

Define an unsafe function that is const as long as libc_const_extern_fn is enabled.