[libsigc++2/variadic_bind] Use decltype(auto).

[Murray Cumming <murrayc src gnome org>]

commit 3a55cd4e13ec21d5c39411e8d26cea945f8db47d
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Feb 19 11:32:26 2016 +0100

    Use decltype(auto).

 sigc++/adaptors/bind.h |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/sigc++/adaptors/bind.h b/sigc++/adaptors/bind.h
index b054277..3543703 100644
--- a/sigc++/adaptors/bind.h
+++ b/sigc++/adaptors/bind.h
@@ -94,7 +94,8 @@ struct TransformEachInvoker
 {
   //We take T_element as non-const because invoke() is not const.
   static
-  auto transform(T_element& element) -> decltype(element.invoke())& {
+  decltype(auto)
+  transform(T_element& element) {
     return element.invoke();
   }
 };