VideoCapture is not working in OpenCV 2.4.2

edit

I recently installed OpenCV 2.4.2 in Ubuntu 12.04.

cap = VideoCapture(0)

is working. but I can't grab frames from some video source.

cap = VideoCapture("input.avi")
img = cap.read()

gives me a numpy with all zero elements.

I have also installed ffmpeg 0.11, Latest snapshot of x264, v4l-0.8.8 (All are latest stable versions)

cmake -D WITH_QT=ON -D WITH_FFMPEG=ON -D WITH_OPENGL=ON -D WITH_TBB=ON -D BUILD_EXAMPLES=OFF WITH_V4L=ON .. 
make
sudo make install

When I do cmake, I get this

-- Detected version of GNU GCC: 46 (406) -- Found OpenEXR: /usr/lib/libIlmImf.so

-- Looking for linux/videodev.h

-- Looking for linux/videodev.h - not found

-- Looking for linux/videodev2.h

-- Looking for linux/videodev2.h - found

-- Looking for libavformat/avformat.h

-- Looking for libavformat/avformat.h - found

-- Looking for ffmpeg/avformat.h

-- Looking for ffmpeg/avformat.h - not found

-- checking for module 'tbb'

-- package 'tbb' not found

And

-- Video I/O:

-- DC1394 1.x: NO

-- DC1394 2.x: YES (ver 2.2.0)

-- FFMPEG: YES

-- codec: YES (ver 54.23.100)

-- format: YES (ver 54.6.100)

-- util: YES (ver 51.54.100)

-- swscale: YES (ver 2.1.100)

-- gentoo-style: YES

-- GStreamer:

-- base: YES (ver 0.10.36)

-- app: YES (ver 0.10.36)

-- video: YES (ver 0.10.36)

-- OpenNI: NO

-- OpenNI PrimeSensor Modules: NO

-- PvAPI: NO

-- UniCap: NO

-- UniCap ucil: NO

-- V4L/V4L2: Using libv4l (ver 0.8.8)

-- XIMEA: NO

-- Xine: NO

What's the problem here ?