message: Add "apple CROSSTOOL" to common problems
diff: @@ -441,6 +441,15 @@ Stack Overflow and specify the `tensorflow` tag.
  <td>Invoking `python` or `ipython` generates the following error:
  <pre>ImportError: cannot import name pywrap_tensorflow</pre></td>
  </tr>
  +
  +<tr>
  + <td><a href="https://stackoverflow.com/questions/45276830">45276830</a></td>
  + <td><pre>external/local_config_cc/BUILD:50:5: in apple_cc_toolchain rule
  + @local_config_cc//:cc-compiler-darwin_x86_64: Xcode version must be specified
  + to use an Apple CROSSTOOL.</pre>
  + </td>
  +</tr>
  +
  </table>
  ## Tested source configurations
  