title: I try to build webrtc in using xcode,but missing rtc_base_approved_generic.a,How to fix it?
body: <pre><code>build rtc_base_approved_generic: phony obj/rtc_base/rtc_base_approved_generic.stamp
  </code></pre>
  
  <p>what is <code>rtc_base_approved_generic.stamp</code> and what is it used for?
  <code>rtc_base_approved_generic.stamp</code> is empty，why the script does not create <code>rtc_base_approved_generic.a</code>. Will appreciated for any help.</p>