Strip empty strings from link args

Closes rust-lang#15487

src/librustc/metadata/cstore.rs

@@ -196,7 +196,7 @@ impl CStore {
     }
 
     pub fn add_used_link_args(&self, args: &str) {
-        for s in args.split(' ') {
+        for s in args.split(' ').filter(|s| !s.is_empty()) {
             self.used_link_args.borrow_mut().push(s.to_string());
         }
     }

src/test/run-pass/issue-15487.rs

@@ -0,0 +1,20 @@
+// Copyright 2014 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.
+
+// ignore-win32
+
+#![feature(link_args)]
+
+#[link_args="-lc  -lm"]
+#[link_args=" -lc"]
+#[link_args="-lc "]
+extern {}
+
+fn main() {}