[gtkmm] Use C++14

[Murray Cumming <murrayc src gnome org>]

commit 38987e708548c90f81929e43c8d1f0db0959edf6
Author: Murray Cumming <murrayc murrayc com>
Date:   Tue Mar 8 12:46:58 2016 +0100

    Use C++14

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 5c58f5b..387b8e9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -45,7 +45,7 @@ MM_CONFIG_DOCTOOL_DIR([docs])
 AC_SUBST([LIBGTKMM_SO_VERSION], [0:0:0])
 
 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])