Check for conflicts between macros 1.0 exports (#[macro_export], #[macro_reexport])

and macros 2.0 exports (`pub use` macro re-exports and `pub macro` (once implemented)
at the crate root.

src/test/compile-fail/duplicate-check-macro-exports.rs

@@ -0,0 +1,19 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(use_extern_macros)]
+
+pub use std::panic; //~ NOTE previous macro export here
+
+#[macro_export]
+macro_rules! panic { () => {} } //~ ERROR a macro named `panic` has already been exported
+//~| NOTE `panic` already exported
+
+fn main() {}