[gtksourceviewmm] Build: Require C++17

[Murray Cumming <murrayc src gnome org>]

commit fabfb3d5b91518063e8f8414513101813b4a03a8
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Sep 27 22:54:42 2019 +0200

    Build: Require C++17
    
    Because it is required by gtkmm-4.0.

 configure.ac | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 8fd454d..a0ee62a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -13,7 +13,7 @@ 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([0.9.8])
+MM_PREREQ([0.9.12])
 MM_INIT_MODULE([gtksourceviewmm-5.0])
 
 # Copy the mm-common .pl scripts into docs/,
@@ -25,7 +25,7 @@ AC_SUBST([GTKSOURCEVIEWMM_SO_VERSION], [0:0:0])
 
 LT_INIT([win32-dll disable-static])
 AC_PROG_CXX
-MM_AX_CXX_COMPILE_STDCXX_11([noext],[mandatory])
+MM_AX_CXX_COMPILE_STDCXX([17], [noext],[mandatory])
 
 AC_SUBST([GTKSOURCEVIEWMM_MODULES], ['gtkmm-4.0 >= 3.18.0 gtksourceview-4 >= 3.99.0'])
 PKG_CHECK_MODULES([GTKSOURCEVIEWMM], [$GTKSOURCEVIEWMM_MODULES])