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
@tSed could you make the changes and push them to the branch?