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
This change does not allow me to build OpenCV with Gstreamer 0.10 support if both versions are available.