[gtkmm-documentation] Require C++17

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 8b90c6cd456d7596cc02299c6d475f86fbc3f46a
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Mon Apr 16 15:27:35 2018 +0200

    Require C++17
    
    Because libsigc++-3.0 (and therefore gtkmm-4.0) now requires C++17.
    Require glibmm-2.58 >= 2.57.1 and gtkmm-4.0 >= 3.93.0

 configure.ac |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index f3dc26a..9aa75f4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -17,16 +17,16 @@ 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])
 
 AC_PROG_CXX
-MM_AX_CXX_COMPILE_STDCXX([14], [noext], [mandatory])
+MM_AX_CXX_COMPILE_STDCXX([17], [noext], [mandatory])
 
 AC_PATH_PROG([PERL], [perl], [perl])
 AC_PROG_SED
 
-PKG_CHECK_MODULES([GIOMM], [giomm-2.58 >= 2.55.1])
-PKG_CHECK_MODULES([GTKMM], [gtkmm-4.0  >= 3.91.2])
+PKG_CHECK_MODULES([GIOMM], [giomm-2.58 >= 2.57.1])
+PKG_CHECK_MODULES([GTKMM], [gtkmm-4.0  >= 3.93.0])
 MM_PKG_CONFIG_SUBST([GTHREAD_CFLAGS], [--cflags-only-other gthread-2.0])
 
 # glib-compile-resources and C compiler are needed to generate and compile