[CMake] FindGTK3.cmake should not modify the values of build options

[CMake] FindGTK3.cmake should not modify the values of build options
https://bugs.webkit.org/show_bug.cgi?id=144613

Reviewed by Martin Robinson.

* Source/cmake/FindGTK3.cmake: Do not check or set the value of ENABLE_X11_TARGET and
ENABLE_WAYLAND_TARGET. Instead, simply define the variables GTK3_SUPPORTS_X11 and
GTK3_SUPPORTS_WAYLAND as appropriate. Also, rename GTK_SUPPORTS_GESTURES to
GTK3_SUPPORTS_GESTURES for consistency.
* Source/cmake/OptionsGTK.cmake: Fail the build if the appropriate GTK+ backend is not
available. It's not possible to automatically select a backend correctly anymore, since all
options are set at the same time.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@185788 268f45cc-cd09-0410-ab3c-d52691b4dbfc