fix memory leaks in JSON.generate

Test Script:
{{{
require 'json'

loop do
  JSON.generate("key1" => "foo", "key2" => "bar")
end
}}}

ext/json/generator/extconf.rb

@@ -17,4 +17,5 @@
   have_header("ruby/re.h")
   have_header("ruby/encoding.h")
 end
+$INCFLAGS << ' -I../../..'
 create_makefile 'json/ext/generator'

ext/json/generator/generator.c

@@ -1315,6 +1315,24 @@ static VALUE cState_depth_set(VALUE self, VALUE depth)
     return state->depth = FIX2LONG(depth);
 }
 
+#ifdef __MACRUBY__
+#include "macruby_internal.h"
+#include "objc.h"
+static IMP json_generator_state_finalize_imp_super = NULL;
+static void
+json_generator_state_finalize_imp(void *self, SEL sel)
+{
+    JSON_Generator_State *objState;
+    Data_Get_Struct(self, JSON_Generator_State, objState);
+    if (objState != NULL) {
+	State_free(objState);
+    }
+    if (json_generator_state_finalize_imp_super != NULL) {
+	((void(*)(void *, SEL))json_generator_state_finalize_imp_super)(self, sel);
+    }
+}
+#endif
+
 /*
  *
  */
@@ -1416,4 +1434,9 @@ void Init_generator()
 #endif
     i_SAFE_STATE_PROTOTYPE = rb_intern("SAFE_STATE_PROTOTYPE");
     CJSON_SAFE_STATE_PROTOTYPE = Qnil;
+
+#ifdef __MACRUBY__
+    json_generator_state_finalize_imp_super = rb_objc_install_method2((Class)cState,
+	    "finalize", (IMP)json_generator_state_finalize_imp);
+#endif
 }

ext/json/generator/generator.h

@@ -7,7 +7,7 @@
 
 #include "ruby.h"
 
-#if WITH_OBJC
+#ifdef __MACRUBY__
 /* We cannot use the GC memory functions here because the underlying libedit
  * function will call free() on the memory, resulting in a leak.
  */