title: Two libstdc++.so.6 files and version `GLIBCXX_3.4.21' not found error
body: <p>I have Ubuntu 15.10 with GCC 5.2.1 and Qt 5.5.1.
  When I try to run my Qt application, I get this error:</p>
  
  <blockquote>
    <p>~/Qt/builds/gvis-Desktop_Qt_5_5_1_GCC_64bit-Debug/app/gvis: ~/Qt/5.5/gcc_64/lib/libstdc++.so.6: version `GLIBCXX_3.4.21' not found</p>
  </blockquote>
  
  <p><code>strings</code> command shows that <code>/usr/lib/x86_64-linux-gnu/libstdc++.so.6</code> file has <code>GLIBCXX_3.4.21</code> and <code>~/Qt/5.5/gcc_64/lib/libstdc++.so.6</code> hasn't.</p>
  
  <p>I tried to replace qt libstdc++.so file with system one but got a lot of errors in runtime.</p>
  
  <p>Why there are another libstdc++.so file in Qt SDK directory? And how to get my app running again? It worked well on Ubuntu 15.04.</p>
  
  <p>(Please don't tell me that I should downgrade my Ubuntu or Qt SDK)</p>