[goocanvasmm] Build: Require C++14, because sigc++-3.0 does.

[Murray Cumming <murrayc src gnome org>]

commit 0d55fd0eb3018e6225d5dcee97dd9792a7efa7e3
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon Jun 19 14:24:29 2017 +0200

    Build: Require C++14, because sigc++-3.0 does.

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 5a2f9cd..7f353a3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -40,7 +40,7 @@ AC_SUBST([GOOCANVASMM_SO_VERSION], [6:0:0])
 
 AC_PROG_CXX
 
-MM_AX_CXX_COMPILE_STDCXX_11([noext],[mandatory])
+MM_AX_CXX_COMPILE_STDCXX([14], [noext],[mandatory])
 
 AC_DISABLE_STATIC
 AC_LIBTOOL_WIN32_DLL