[atkmm] Use glibmm-2.56 instead of glibmm-2.54.

[Murray Cumming <murrayc src gnome org>]

commit e4cf3cfa96aa0fd86e17f78cfecd4c4a5a8ac9b3
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Aug 25 12:12:15 2017 +0200

    Use glibmm-2.56 instead of glibmm-2.54.
    
    We have changed the ABI name in glibmm.

 codegen/Makefile.am |    2 +-
 configure.ac        |    6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)
---
diff --git a/codegen/Makefile.am b/codegen/Makefile.am
index 1d82ba3..d46fbc4 100644
--- a/codegen/Makefile.am
+++ b/codegen/Makefile.am
@@ -30,7 +30,7 @@ dist_noinst_SCRIPTS = generate_defs_and_docs.sh
 noinst_PROGRAMS  = extradefs/generate_extra_defs
 
 extradefs_generate_extra_defs_SOURCES = extradefs/generate_extra_defs_atk.cc
-extradefs_generate_extra_defs_LDADD   = $(ATKMM_LIBS) -lglibmm_generate_extra_defs-2.54
+extradefs_generate_extra_defs_LDADD   = $(ATKMM_LIBS) -lglibmm_generate_extra_defs-2.56
 extradefs_generate_extra_defs_LDFLAGS = -no-undefined -avoid-version
 
 AM_CPPFLAGS = -I$(top_builddir) $(GTHREAD_CFLAGS) $(ATKMM_CFLAGS)
diff --git a/configure.ac b/configure.ac
index 2ba84df..60ae5ef 100644
--- a/configure.ac
+++ b/configure.ac
@@ -51,18 +51,18 @@ MM_AX_CXX_COMPILE_STDCXX([14], [noext], [mandatory])
 AC_DISABLE_STATIC
 LT_INIT([win32-dll])
 
-AC_SUBST([ATKMM_MODULES], ['atk >= 1.18 glibmm-2.54 >= 2.53.1.1'])
+AC_SUBST([ATKMM_MODULES], ['atk >= 1.18 glibmm-2.56 >= 2.53.1.1'])
 PKG_CHECK_MODULES([ATKMM], [$ATKMM_MODULES])
 
 MM_PKG_CONFIG_SUBST([GTHREAD_CFLAGS], [--cflags-only-other gthread-2.0])
-MM_PKG_CONFIG_SUBST([GMMPROC_DIR], [--variable=gmmprocdir glibmm-2.54])
+MM_PKG_CONFIG_SUBST([GMMPROC_DIR], [--variable=gmmprocdir glibmm-2.56])
 
 MM_ARG_DISABLE_DEPRECATED_API
 
 MM_ARG_ENABLE_DOCUMENTATION
 MM_ARG_WITH_TAGFILE_DOC([libstdc++.tag], [mm-common-libstdc++])
 MM_ARG_WITH_TAGFILE_DOC([libsigc++-3.0.tag], [sigc++-3.0])
-MM_ARG_WITH_TAGFILE_DOC([glibmm-2.54.tag], [glibmm-2.54])
+MM_ARG_WITH_TAGFILE_DOC([glibmm-2.56.tag], [glibmm-2.56])
 
 AC_LANG([C++])
 MM_ARG_ENABLE_WARNINGS([ATKMM_WXXFLAGS],