run _6to5-node using execvp() if available

bin/6to5-node

@@ -5,7 +5,6 @@
  * when found, before invoking the "real" _6to5-node(1) executable.
  */
 
-var spawn = require("child_process").spawn;
 var args  = ["--harmony", __dirname + "/_6to5-node"];
 
 process.argv.slice(2).forEach(function(arg){
@@ -49,13 +48,21 @@ process.argv.slice(2).forEach(function(arg){
   }
 });
 
-var proc = spawn(process.argv[0], args, { stdio: "inherit" });
-proc.on("exit", function (code, signal) {
-  process.on("exit", function () {
-    if (signal) {
-      process.kill(process.pid, signal);
-    } else {
-      process.exit(code);
-    }
+try {
+  var kexec = require("kexec");
+  kexec(process.argv[0], args);
+} catch(e) {
+  if (e.code !== "MODULE_NOT_FOUND") throw err;
+  
+  var child_process = require("child_process");
+  var proc = child_process.spawn(process.argv[0], args, { stdio: "inherit" });
+  proc.on("exit", function (code, signal) {
+    process.on("exit", function () {
+      if (signal) {
+        process.kill(process.pid, signal);
+      } else {
+        process.exit(code);
+      }
+    })
   });
-});
+}