[atkmm] Build: Require C++14 instead of C++11.

[Murray Cumming <murrayc src gnome org>]

commit ef717520b38d64ea191b24b365c7a8c915d82436
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon Nov 14 10:46:22 2016 +0100

    Build: Require C++14 instead of C++11.
    
    Because we now use a glibmm that requires C++14.

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 40534f9..958b9df 100644
--- a/configure.ac
+++ b/configure.ac
@@ -46,7 +46,7 @@ MM_CONFIG_DOCTOOL_DIR([doc])
 AC_SUBST([ATKMM_SO_VERSION], [2:0:1])
 
 AC_PROG_CXX
-MM_AX_CXX_COMPILE_STDCXX([11], [noext], [mandatory])
+MM_AX_CXX_COMPILE_STDCXX([14], [noext], [mandatory])
 
 AC_DISABLE_STATIC
 LT_INIT([win32-dll])