cmake: fix gstreamer check conditions

Hi all,

Here is a change fixing the Gstreamer detection and the way OpenCV honor its configure option wrt Gstreamer support.

Further details in the commit message.

Note that , thought this patch has been done against the 2.4 branch, it can be applied with no conflict on master branch. 

Regards,

Samuel
Gotcha!

@asmorkalov : the behavior you experienced is caused by the option setting `WITH_GSTREAMER` to `ON` by default.

So, when one wants to link OpenCV with gstreamer-0.10, he/she should set both flags, for disabling gstreamer-1.x and enabling gstreamer-0.10.

IMHO, this is more a mis-configuration of OpenCV, or a lack of documentation, than a bug in the gstreamer detection.

Maybe OpenCV should bail out when both `WITH_GSTREAMER` and `WITH_GSTREAMER_0_10` are `ON`? 

This raises the question: what gstreamer version the OpenCV community wants to promote? (so, what is the default OpenCV's behavior wrt gstreamer?).