title: Opencv, undefined reference to `cv::imread(??cv::String const&???, int)'
body: <p>I have a project in QT + Opencv, the code was working but I had to format the windows and now I'm trying to import the project again and this error appeared.</p>
  
  <p>undefined reference to `cv::imread(cv::String const&amp;, int)'
  in this line:</p>
  
  <pre><code> mat = cv::imread(path);
  </code></pre>
  
  <p>although my code looks like this:</p>
  
  <pre><code> QString caminho = QFileDialog::getOpenFileName(this, tr("Open File"),
                                         "",
                                         tr("Images (*.png *.tiff)") );
  
   std::string path = caminho.toStdString();
   mat = cv::imread(path);
  </code></pre>
  
  <p>I tried other string like codes like "image.png" and did not work.</p>
  
  <p>Using OpenCV3, QT5.4, Mingw, Windows8.1</p>
  
  <p>sorry my english</p>