Add a nice error message when json failes to parse and fix Makefile rule

I had a json that wouldn't parse (trailing comma) but it was an
odd php error about iterating over a null.  So this should give a
slightly better error.

Also, .inc files are no longer used.  Remove the Makefile references.

hphp/idl/base.php

@@ -370,6 +370,9 @@ function DefineFunction($func) {
 
 function ReadIDLFile($path) {
   $entries = json_decode(file_get_contents($path), /* use arrays */ true);
+  if (!$entries) {
+    throw new Exception("Unable to parse json from $path");
+  }
 
   if (!empty($entries['funcs'])) {
     foreach ($entries['funcs'] as $func) {