title: Mojave + Xcode 10 build fails on glog config.h, gflags/gflags.h
body: <p>I'm testing React Native 0.56.0-rc.2 on Mac OS Mojave and Xcode 10.</p>
  
  <p>Running:</p>
  
  <pre><code>react-native init TestProject --version="0.56.0-rc.2"
  cd TestProject
  npm run start
  react-native run-ios
  </code></pre>
  
  <p>Which generated some long errors.</p>
  
  <p>Entering Xcode gave me a failure on missing <code>config.h</code> for glog, which I found could be built manually:</p>
  
  <pre><code>cd ./node_modules/react-native/third-party/glog-0.3.4
  ./configure &amp;&amp; make &amp;&amp; make install
  </code></pre>
  
  <p>Which passed that stage, but got me on yet another issue</p>
  
  <pre><code>(...)/node_modules/react-native/third-party/glog-0.3.4/src/glog/logging.h:85:10:
  'gflags/gflags.h' file not found
  </code></pre>
  
  <p>I can't find anything related to RN and this on Google or Stack Overflow, only references to other packages and instructions to install those on a Debian-based system.</p>
  
  <p>Is this a known issue?</p>