[libxml++] Build: Use glibmm-2.56 instead of glibmm-2.54.

[Murray Cumming <murrayc src gnome org>]

commit a56bfed2fab263aa127866520db622fc081d2d9c
Author: Murray Cumming <murrayc murrayc com>
Date:   Sun Oct 22 21:33:27 2017 +0200

    Build: Use glibmm-2.56 instead of glibmm-2.54.
    
    We changed the (unstable) ABI name in glibmm a while ago.

 configure.ac |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 097ccf7..f46f2e6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -35,7 +35,7 @@ MM_AX_CXX_COMPILE_STDCXX([11], [noext],[mandatory])
 LT_PREREQ([2.2.6])
 LT_INIT([dlopen win32-dll disable-static])
 
-AC_SUBST([LIBXMLXX_MODULES], ['libxml-2.0 >= 2.7.7 glibmm-2.54 >= 2.32.0'])
+AC_SUBST([LIBXMLXX_MODULES], ['libxml-2.0 >= 2.7.7 glibmm-2.56 >= 2.32.0'])
 PKG_CHECK_MODULES([LIBXMLXX], [$LIBXMLXX_MODULES])
 
 AC_LANG([C++])
@@ -45,7 +45,7 @@ LIBXMLXX_CXX_HAS_EXCEPTION_PTR
 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])
 
 # Evaluate the --enable-warnings=level option.
 MM_ARG_ENABLE_WARNINGS([LIBXMLXX_WXXFLAGS],