[gtkmm] Require gtk+ 2.20.0

[Murray Cumming <murrayc src gnome org>]

commit 1d1eaaa594bc8414da37c3909ba91f975f5299d9
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon Mar 21 20:41:57 2016 +0100

    Require gtk+ 2.20.0

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 4c50d2c..ae4dbfc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -93,7 +93,7 @@ AS_IF([test "x$enable_static" = xyes],
 # gdkmm really does need GTK+, because part of Gdk::DragContext is in GTK+.
 # The extra cairomm-1.0 check is because gdkmm requires a newer cairomm than pangomm requires.
 # The extra gdk-pixbuf-2.0 check is because gdkmm requires a newer gdk-pixbuf than gtk+-3.0 requires.
-AC_SUBST([GDKMM_MODULES], ['giomm-2.4 >= 2.47.3.1 pangomm-1.4 >= 2.38.2 gtk+-3.0 >= 3.19.2 cairomm-1.0 >= 
1.12.0 gdk-pixbuf-2.0 >= 2.26.0'])
+AC_SUBST([GDKMM_MODULES], ['giomm-2.4 >= 2.47.3.1 pangomm-1.4 >= 2.38.2 gtk+-3.0 >= 3.20.0 cairomm-1.0 >= 
1.12.0 gdk-pixbuf-2.0 >= 2.26.0'])
 
 AS_IF([test "x$gtkmm_host_windows" = xyes],
       [GTKMM_MODULES=$GDKMM_MODULES],