Correct error with GTK3 not found selection

When GTK3 is not found, HAVE_GTK was being set to TRUE. This edit
ensures that HAVE_GTK is only set if GTK3 or GTK2 (meeting minimum
version requirements) is present.
Selection logic for printing 'GTK: No' when the libraries are not found
has also been removed so the message is printed when GTK is not found or
selected in common with other libraries.

Changes committed:
	modified:   CMakeLists.txt
	modified:   cmake/OpenCVFindLibsGUI.cmake