Update built-in help for --extern.
diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index 1740d8f..b128428 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -1800,7 +1800,7 @@
             "",
             "extern",
             "Specify where an external rust library is located",
-            "NAME=PATH",
+            "NAME[=PATH]",
         ),
         opt::multi_s(
             "",