source_spellcheck: fixed lambda support for signals on older platforms

src/source_spellcheck.cc

@@ -2,6 +2,20 @@
 #include "config.h"
 #include <iostream>
 
+namespace sigc {
+#ifndef SIGC_FUNCTORS_DEDUCE_RESULT_TYPE_WITH_DECLTYPE
+  template <typename Functor>
+  struct functor_trait<Functor, false> {
+    typedef decltype (::sigc::mem_fun(std::declval<Functor&>(),
+                                      &Functor::operator())) _intermediate;
+    typedef typename _intermediate::result_type result_type;
+    typedef Functor functor_type;
+  };
+#else
+  SIGC_FUNCTORS_DEDUCE_RESULT_TYPE_WITH_DECLTYPE
+#endif
+}
+
 AspellConfig* Source::SpellCheckView::spellcheck_config=NULL;
 
 Source::SpellCheckView::SpellCheckView() : Gsv::View() {