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
@asmorkalov : to be honest, I don't see how the change on line 16 prevent you from building OpenCV with gstreamer-0.10 :-/

I've just rechecked here, it works fine.

On the CMake command line, did you only specify `-DWITH_GSTREAMER_0_10=ON` (not `-DGSTREAMER=OFF`).?
I run all the tests setting both flags.