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.c.template

@@ -0,0 +1,60 @@
+/*** 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 ***/
+/* enumerations from "@filename@" */
+#include "@filename@"
+
+/*** END file-production ***/
+
+/*** BEGIN value-header ***/
+GType
+@enum_name@_get_type (void)
+{
+  static volatile gsize the_type__volatile = 0;
+
+  if (g_once_init_enter (&the_type__volatile))
+    {
+      static const G@Type@Value values[] = {
+/*** END value-header ***/
+
+/*** BEGIN value-production ***/
+        { @VALUENAME@,
+          "@VALUENAME@",
+          "@valuenick@" },
+/*** END value-production ***/
+
+/*** BEGIN value-tail ***/
+        { 0, NULL, NULL }
+      };
+
+      GType the_type = g_@type@_register_static (
+        g_intern_static_string ("@EnumName@"),
+        values);
+
+      g_once_init_leave (&the_type__volatile, the_type);
+    }
+
+  return the_type__volatile;
+}
+
+/*** END value-tail ***/