`#[macro_use]` does not work on macro-expanded `extern crate`s

For example, this compiles:

``` rust
#![feature(rustc_private)]

#[macro_use]
extern crate syntax;

fn main() {
    help!(S, "");

    struct S;
    impl S { fn help(&self, _: &str) {} }
}
```

but this doesn't:

``` rust
#![feature(rustc_private)]

macro_rules! my_extern_crate {
    ($i:ident) => { #[macro_use] extern crate $i; }
}
my_extern_crate!(syntax);

fn main() { ... }
```
