title: libaom Cmake(Ubuntu18.04) failed
body: <p>I am compiling aom in Ubuntu18.04 with the code commands are as below:<br />
  ・libaom</p>
  <pre><code>mkdir aom-dev  
  cd aom-dev  
  git clone https://aomedia.googlesource.com/aom  
  cmake -DCONFIG_PIC=1 ./aom  
  make -j4  
  sudo make install
  </code></pre>
  <p>error message outputed:</p>
  <pre><code>Checking C compiler flag support for: -Wextra-semi  
  -- Performing Test C_FLAG_SUPPORTED  
  -- Performing Test C_FLAG_SUPPORTED - Failed  
  Checking C++ compiler flag support for: -Wextra-semi  
  -- Performing Test CXX_FLAG_SUPPORTED  
  -- Performing Test CXX_FLAG_SUPPORTED - Failed
  </code></pre>
  <p>Any hint?</p>