[libsigc++2] Require C++11

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 5a6b8f21f83dfe0f64770196e9124e4763288489
Author: Kjell Ahlstedt <kjell ahlstedt bredband net>
Date:   Tue Jul 14 16:29:47 2015 +0200

    Require C++11
    
    * configure.ac: Use MM_AX_CXX_COMPILE_STDCXX_11 to check for compiler
    support for C++11 and use it (--std=c++11 for current versions of
    g++). Bug #672555.

 configure.ac |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 06b140e..2387002 100644
--- a/configure.ac
+++ b/configure.ac
@@ -30,11 +30,14 @@ m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES])
 AM_MAINTAINER_MODE
 AC_ARG_VAR([ACLOCAL_FLAGS], [aclocal flags, e.g. -I <macro dir>])
 
+#TODO: Require mm-common 0.9.8 when it is available
 MM_PREREQ([0.9.7])
 MM_INIT_MODULE([sigc++-2.0])
 MM_CONFIG_DOCTOOL_DIR([docs])
 
 AC_PROG_CXX
+MM_AX_CXX_COMPILE_STDCXX_11([noext],[mandatory])
+
 AC_DISABLE_STATIC
 LT_INIT([win32-dll])