[gtkmm-documentation] Use glibmm-2.58 instead of glibmm-2.56

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 650a1a05848bd3854de8d4c9f760c03c651ee12c
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Mon Apr 2 09:59:35 2018 +0200

    Use glibmm-2.58 instead of glibmm-2.56
    
    We have changed the ABI name in glibmm.

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 98a1d51..f3dc26a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -25,7 +25,7 @@ MM_AX_CXX_COMPILE_STDCXX([14], [noext], [mandatory])
 AC_PATH_PROG([PERL], [perl], [perl])
 AC_PROG_SED
 
-PKG_CHECK_MODULES([GIOMM], [giomm-2.56 >= 2.55.1])
+PKG_CHECK_MODULES([GIOMM], [giomm-2.58 >= 2.55.1])
 PKG_CHECK_MODULES([GTKMM], [gtkmm-4.0  >= 3.91.2])
 MM_PKG_CONFIG_SUBST([GTHREAD_CFLAGS], [--cflags-only-other gthread-2.0])