[gtkmm] Require C++17

[Murray Cumming <murrayc src gnome org>]

commit d967943dc7ebd2b3b44c8f34fa6601b4a3351e90
Author: Murray Cumming <murrayc murrayc com>
Date:   Sun Apr 8 22:10:35 2018 +0200

    Require C++17
    
    Because libsigc++-3.0 (and therefore glibmm) now requires C++17.

 configure.ac |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 3bebf53..f144965 100644
--- a/configure.ac
+++ b/configure.ac
@@ -32,7 +32,7 @@ m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 AM_MAINTAINER_MODE
 AC_ARG_VAR([ACLOCAL_FLAGS], [aclocal flags, e.g. -I <macro dir>])
 
-MM_PREREQ([0.9.10])
+MM_PREREQ([0.9.12])
 MM_INIT_MODULE([gdkmm-4.0])
 MM_INIT_MODULE([gtkmm-4.0])
 
@@ -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([14], [noext],[mandatory])
+MM_AX_CXX_COMPILE_STDCXX([17], [noext],[mandatory])
 
 AC_DISABLE_STATIC
 LT_INIT([win32-dll])