implement #[nolink]; deprecate #[link_name = ""]; note in stdlib to remove empty link_name.

Can't remove them from stdlib until the snapshotted compiler supports #[nolink].

src/comp/metadata/creader.rs

@@ -57,12 +57,20 @@ fn visit_item(e: env, i: @ast::item) {
           }
           either::left(msg) { e.sess.span_fatal(i.span, msg); }
         }
+
         let cstore = e.sess.get_cstore();
         let native_name = i.ident;
+        if vec::len(attr::find_attrs_by_name(i.attrs, "nolink")) > 0u {
+            ret;
+        }
         alt attr::get_meta_item_value_str_by_name(i.attrs, "link_name") {
           some(nn) { native_name = nn; }
           none. { }
         }
+        if native_name == "" {
+            e.sess.span_fatal(i.span,
+                "empty #[link_name] not allowed; use #[nolink].");
+        }
         if !cstore::add_used_library(cstore, native_name) { ret; }
         for a: ast::attribute in
             attr::find_attrs_by_name(i.attrs, "link_args") {

src/comp/metadata/cstore.rs

@@ -93,7 +93,6 @@ fn add_used_library(cstore: cstore, lib: str) -> bool {
     if lib == "" { ret false; }
 
     if vec::member(lib, p(cstore).used_libraries) { ret false; }
-
     p(cstore).used_libraries += [lib];
     ret true;
 }