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
Please move this block of code inside previous one, like:
```
if(WITH_GSTREAMER AND NOT WITH_GSTREAMER_0_10)
...
# gstreamer support was requested but could not find gstreamer 1.x,
# so fallback/try to find gstreamer 0.10
  if(WITH_GSTREAMER AND NOT HAVE_GSTREAMER)
    set(WITH_GSTREAMER_0_10 ON)
  endif()
endif(WITH_GSTREAMER AND NOT WITH_GSTREAMER_0_10)
```

It allows to use old GStreamer on new systems and enables fall-back mode you want.