[gnome-sudoku] Switch to C++ 11

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 8ba910480917ef11a3ea6c0c3e7a642a28d230b1
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Mon Jul 11 23:02:27 2016 -0500

    Switch to C++ 11
    
    We could go all the way to C++ 14, but we don't need it for anything. I
    want C++ 11 to use std::call_once.

 configure.ac |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 76f55c2..4e132f7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -10,6 +10,8 @@ AM_MAINTAINER_MODE([enable])
 AC_PROG_CC
 AC_PROG_CXX
 
+AX_CXX_COMPILE_STDCXX_11([noext], [mandatory])
+
 AM_PROG_VALAC([0.27.1.30])
 AM_PROG_CC_C_O