Macro cfg_if

macro_rules! cfg_if {
    ($(
        if #[cfg($meta:meta)] { $($tokens:tt)* }
    ) else * else {
        $($tokens2:tt)*
    }) => { ... };
    (
        if #[cfg($i_met:meta)] { $($i_tokens:tt)* }
        $( else if #[cfg($e_met:meta)] { $($e_tokens:tt)* } )*
    ) => { ... };
    (@__items ($($not:meta,)*) ; ) => { ... };
    (@__items ($($not:meta,)*) ; ( ($($m:meta),*) ($($tokens:tt)*) ), $($rest:tt)*) => { ... };
    (@__identity $($tokens:tt)*) => { ... };
}

libm cannot have dependencies, so this is vendored directly from the cfg-if crate (with some comments stripped for compactness).