simplify yajl related code

Specifically, put the version dependent code in some macros, and put
that plus the `y' and `ystr' macros in a separate file, `yajl_utils.h'.

include/yajl_utils.h

@@ -0,0 +1,31 @@
+/*
+ * vim:ts=4:sw=4:expandtab
+ *
+ * i3 - an improved dynamic tiling window manager
+ * © 2009-2011 Michael Stapelberg and contributors (see also: LICENSE)
+ *
+ * yajl_utils.h
+ *
+ */
+#ifndef I3_YAJL_UTILS_H
+#define I3_YAJL_UTILS_H
+
+#include <yajl/yajl_gen.h>
+#include <yajl/yajl_parse.h>
+#include <yajl/yajl_version.h>
+
+/* Shorter names for all those yajl_gen_* functions */
+#define y(x, ...) yajl_gen_ ## x (gen, ##__VA_ARGS__)
+#define ystr(str) yajl_gen_string(gen, (unsigned char*)str, strlen(str))
+
+#if YAJL_MAJOR >= 2
+#define ygenalloc() yajl_gen_alloc(NULL)
+#define yalloc(callbacks, client) yajl_alloc(callbacks, NULL, client)
+typedef size_t ylength;
+#else
+#define ygenalloc() yajl_gen_alloc(NULL, NULL);
+#define yalloc(callbacks, client) yajl_alloc(callbacks, NULL, NULL, client)
+typedef unsigned int ylength;
+#endif
+
+#endif