configure: bump gtk-doc req to 1.12 (mar-2009)

This allows us to e.g. unconditionally use gtkdoc-rebase.
diff --git a/configure.ac b/configure.ac
index 4ac9af6..a2b24d8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -152,7 +152,7 @@
 GOBJECT_INTROSPECTION_CHECK([1.31.1])
 
 dnl check for documentation tools
-GTK_DOC_CHECK([1.3])
+GTK_DOC_CHECK([1.12])
 AS_PATH_PYTHON([2.1])
 AG_GST_PLUGIN_DOCS([1.3],[2.1])