[libsigc++2/variadic_bind] partial build fix

[Murray Cumming <murrayc src gnome org>]

commit a730a9dee7ad4c0f48ee8d208811301eedfef33a
Author: Marcin Kolny <marcin kolny gmail com>
Date:   Mon Jan 18 23:29:47 2016 +0100

    partial build fix

 sigc++/adaptors/bind.h        |    3 +--
 sigc++/tuple_transform_each.h |    4 ++--
 2 files changed, 3 insertions(+), 4 deletions(-)
---
diff --git a/sigc++/adaptors/bind.h b/sigc++/adaptors/bind.h
index 2ff23b7..9dda276 100644
--- a/sigc++/adaptors/bind.h
+++ b/sigc++/adaptors/bind.h
@@ -94,8 +94,7 @@ struct TransformEachInvoker
 {
   //We take T_element as non-const because invoke() is not const.
   static
-  decltype(auto)
-  transform(T_element& element) {
+  auto transform(T_element& element) -> decltype(element.invoke())& {
     return element.invoke();
   }
 };
diff --git a/sigc++/tuple_transform_each.h b/sigc++/tuple_transform_each.h
index d64fe85..a55a485 100644
--- a/sigc++/tuple_transform_each.h
+++ b/sigc++/tuple_transform_each.h
@@ -99,7 +99,7 @@ struct tuple_transform_each_impl
     using element_type = typename std::tuple_element<index, T_current>::type;
 
     auto& from = std::get<index>(t_original);
-    const auto element = T_transformer<element_type>::transform(from);
+    const auto& element = T_transformer<element_type>::transform(from);
     const auto t_element = std::make_tuple(element);
     
     const auto t_start = tuple_start<index>(t);
@@ -131,7 +131,7 @@ struct tuple_transform_each_impl<T_transformer, 0>
     constexpr std::size_t index = 0;
 
     using element_type = typename std::tuple_element<index, T_original>::type;
-    const auto element = T_transformer<element_type>::transform(std::get<index>(t_original));
+    const auto& element = T_transformer<element_type>::transform(std::get<index>(t_original));
     const auto tuple_element = std::make_tuple(element);
     const auto tuple_rest = tuple_cdr(t);
     return std::tuple_cat(tuple_element, tuple_rest);