fetcher: Add "config-flags" construct-only property

A lot of effort here just to avoid touching SoupSession directly in
ostree_fetcher_new().  The reason will become apparent in subsequent
commits.

Note this introduces generated enum/flags GTypes using glib-mkenums.
I could have just made the property type as plain integer, but doing
properties right will henceforth be easier now that the automake-fu
is established.

src/libostree/ostree-enumtypes.h.template

@@ -0,0 +1,43 @@
+/*** BEGIN file-header ***/
+/*
+ * Copyright (C) 2015 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+/*** END file-header ***/
+
+/*** BEGIN file-production ***/
+#pragma once
+
+#include <glib-object.h>
+
+G_BEGIN_DECLS
+
+/* Enumerations from "@filename@" */
+
+/*** END file-production ***/
+
+/*** BEGIN enumeration-production ***/
+#define @ENUMPREFIX@_TYPE_@ENUMSHORT@ (@enum_name@_get_type ())
+GType @enum_name@_get_type (void) G_GNUC_CONST;
+
+/*** END enumeration-production ***/
+
+/*** BEGIN file-tail ***/
+G_END_DECLS
+
+/*** END file-tail ***/