[libsigcplusplus/libsigc++-2-10] Fix build of "Implement slot<R(Arg...)> with variadic template"

[Chun-wei Fan <fanchunwei src gnome org>]

commit ac79b551afd80d6874b4f9faefb38aa292ce2159
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Fri Jun 17 14:35:22 2016 +0800

    Fix build of "Implement slot<R(Arg...)> with variadic template"
    
    * sigc++/functors/macros/slot.h.m4: Fix the use of operator() in the
    variadic templates usage in the base slot_call as the syntax differs
    between compilers for this call.  This fixes the build of the tests and
    any items that makes use of sigc++/functors/slot.h on non-GCC,
    specifically Visual Studio 2013 and 2015.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=767777

 sigc++/functors/macros/slot.h.m4 |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/sigc++/functors/macros/slot.h.m4 b/sigc++/functors/macros/slot.h.m4
index e0632e4..60cccbd 100644
--- a/sigc++/functors/macros/slot.h.m4
+++ b/sigc++/functors/macros/slot.h.m4
@@ -475,7 +475,7 @@ struct slot_call
     {
       using typed_slot = typed_slot_rep<T_functor>;
       typed_slot *typed_rep = static_cast<typed_slot*>(rep);
-      return (typed_rep->functor_).template operator()<type_trait_take_t<T_arg>...>
+      return (typed_rep->functor_).SIGC_WORKAROUND_OPERATOR_PARENTHESES<type_trait_take_t<T_arg>...>
                (a_...);
     }