[glibmm] Require C++17.

[Murray Cumming <murrayc src gnome org>]

commit 812000fc9a52d9a77d791bc391767afbee50252a
Author: Murray Cumming <murrayc murrayc com>
Date:   Sun Apr 8 22:03:56 2018 +0200

    Require C++17.
    
    Because libsigc++-3.0 now requires C++17.

 configure.ac |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 8ac2230..5dbcff5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -30,7 +30,7 @@ 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([glibmm-2.58])
 MM_INIT_MODULE([giomm-2.58])
 
@@ -43,7 +43,7 @@ MM_CONFIG_DOCTOOL_DIR([docs])
 AC_SUBST([LIBGLIBMM_SO_VERSION], [4:0:3])
 
 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])