configure: add direct detection of libopencv

The pkg-config file contains all opencv libraries, not only the
neccessary ones.

This change makes it possible to use the libopencv-imgproc-dev Debian
package instead of libopencv-dev, saving about 200 MB of useless
build-dependencies.

In particular one doesn't need to install the parts of opencv that
depend on ffmpeg libraries.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>