Only link librt on linux

Author: brson

src/comp/back/link.rs

@@ -582,7 +582,14 @@ fn link_binary(sess: session::session,
         gcc_args += ["-lm", main];
     }
 
-    gcc_args += ["-lrustrt", "-lrt"];
+    // On linux librt is an indirect dependency via rustrt,
+    // and binutils 2.22+ won't add it automatically
+    if sess.get_targ_cfg().os == session::os_linux {
+        gcc_args += ["-lrt"];
+    }
+
+    // Always want the runtime linked in
+    gcc_args += ["-lrustrt"];
 
     gcc_args += rpath::get_rpath_flags(sess, saved_out_filename);