[CMAKE] Update code to take advantage of CMake version 2.8.3+.
https://bugs.webkit.org/show_bug.cgi?id=97516

Reviewed by NOBODY (OOPS!).

CMake's version was changed to 2.8.3. So, these files below
can be simplified to take advantage of CMake's new version.

* Source/cmake/FindCairo.cmake: Simplified according to CMake 2.8.3+.
* Source/cmake/FindGDK3.cmake: Ditto.
* Source/cmake/FindGStreamer.cmake: Ditto.
* Source/cmake/FindGTK3.cmake: Ditto.
* Source/cmake/FindXt.cmake: Ditto.