title: Install gstreamer support for opencv python package
body: <p>I have built my own opencv python package from source. </p>
  
  <pre><code>import cv2
  print(cv2.__version__)
  </code></pre>
  
  <p>prints: 3.4.5</p>
  
  <p>Now the issue I am facing is regarding the use of gstreamer from the VideoCapture class of opencv. I am trying to get this mimimum working example running on python3</p>
  
  <pre><code>cap = cv2.VideoCapture("videotestsrc ! appsink")
  
  if not cap.isOpened():
      print("Cannot capture test src. Exiting.")
      quit()
  
  while True:
      ret, frame = cap.read()
      if ret == False:
          break
      cv2.imshow("CVtest",frame)
      if cv2.waitKey(1) &amp; 0xFF == ord('q'):
          break
  </code></pre>
  
  <p>Capture fails, producing my print above (see <code>if</code> statement). I checked:</p>
  
  <pre><code>import cv2
  print(cv2.getBuildInformation())
  </code></pre>
  
  <p>prints:</p>
  
  <pre><code>Video I/O:
  DC1394:                      NO
  FFMPEG:                      NO
    avcodec:                   NO
    avformat:                  NO
    avutil:                    NO
    swscale:                   NO
    avresample:                NO
  GStreamer:                   NO
  libv4l/libv4l2:              NO
  v4l/v4l2:                    linux/videodev2.h
  </code></pre>
  
  <p>Seeing that, it made absolute sense that my gstreamer pipeline didn't work. I  ensured WITH_GSTREAMER was set to ON during ccmake of OpenCV (which it already was). Still the issue maintained. I even tried setting WITH_GSTREAMER_0_10 to ON as well. Still no luck having gstreamer enabled from the cv2 python module. </p>
  
  <p>Before anyone suggests using pip3 to install cv2. I tried that, too. The issue with getting the package from pip is, it doesn't let you configure gstreamer support at all. </p>
  
  <p>Can anyone provide help here?</p>