json_pointer: allow the feature to be disabled

Some users may not want to included it in their build/system. So allow a
cmake symbol to disable it.

A user can do 'cmake -DDISABLE_JSON_POINTER=ON <json_c_root_dir>' and
disable the json_pointer functionality. That saves about 17 KB (on an
x86_64) machine. This may be useful on smaller embedded systems; even
though the saving would be fewer kilobytes.

One thing that also needs to change a bit, is that the 'json.h' be
autogenerated via cmake, in order to conditionally include that
"json_pointer.h" file.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

json.h.cmakein

@@ -0,0 +1,38 @@
+/*
+ * $Id: json.h,v 1.6 2006/01/26 02:16:28 mclark Exp $
+ *
+ * Copyright (c) 2004, 2005 Metaparadigm Pte. Ltd.
+ * Michael Clark <michael@metaparadigm.com>
+ * Copyright (c) 2009 Hewlett-Packard Development Company, L.P.
+ *
+ * This library is free software; you can redistribute it and/or modify
+ * it under the terms of the MIT license. See COPYING for details.
+ *
+ */
+
+/**
+ * @file
+ * @brief A convenience header that may be included instead of other individual ones.
+ */
+#ifndef _json_h_
+#define _json_h_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "arraylist.h"
+#include "debug.h"
+#include "json_c_version.h"
+#include "json_object.h"
+#include "json_object_iterator.h"
+@JSON_H_JSON_POINTER@
+#include "json_tokener.h"
+#include "json_util.h"
+#include "linkhash.h"
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif