[metacity] configure.ac: update GTK required version

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 160db8f3f1a57320cbd0c9cfb7f2db3a7354dd8a
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Sat Oct 25 00:43:14 2014 +0300

    configure.ac: update GTK required version
    
    gtk_widget_set_margin_[start/end] are available starting with
    GTK 3.12.0 version.

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 30cb8cb..0c702ad 100644
--- a/configure.ac
+++ b/configure.ac
@@ -109,7 +109,7 @@ fi
 changequote([,])dnl
 
 GTK_API_VERSION=3.0
-GTK_MIN_VERSION=3.8.0
+GTK_MIN_VERSION=3.12.0
 GIO_MIN_VERSION=2.25.10
 CANBERRA_GTK=libcanberra-gtk3