Macro forward_dll::define_function

macro_rules! define_function {
    ($lib:expr, $name:ident, $index:expr, ) => { ... };
    ($lib:expr, $name:ident, $index:expr, $export_name:ident = $proc:ident $($procs:tt)*) => { ... };
    ($lib:expr, $name:ident, $index:expr, $proc:ident $($procs:tt)*) => { ... };
}