specific code. As a result of this, HAVE_GTK3 no longer needs to be
exposed.

The use of HAVE_GTK, and HAVE_ GTK3 have been changed to mirror the
method used by HAVE_QT and HAVE_QT5.
 On branch gtk3
 Changes to be committed:
        modified:   CMakeLists.txt
        modified:   cmake/OpenCVFindLibsGUI.cmake
        modified:   cmake/templates/cvconfig.h.in
        modified:   modules/highgui/src/window.cpp
        modified:   modules/highgui/src/window_gtk.cpp

Conflicts:
	cmake/OpenCVDetectAndroidSDK.cmake
	cmake/OpenCVGenAndroidMK.cmake
	cmake/OpenCVModule.cmake
	cmake/templates/OpenCV.mk.in
	cmake/templates/OpenCVConfig.cmake.in
	doc/tutorials/imgproc/histograms/histogram_comparison/histogram_comparison.rst
	modules/cudabgsegm/src/cuda/mog.cu
	modules/imgproc/perf/opencl/perf_filters.cpp
	modules/imgproc/src/opencl/filterSep_singlePass.cl
	modules/nonfree/CMakeLists.txt
	modules/nonfree/perf/perf_precomp.hpp
	modules/ocl/perf/perf_haar.cpp
	modules/ocl/src/filtering.cpp
	modules/ocl/src/opencl/bgfg_mog.cl
	modules/superres/CMakeLists.txt
	modules/superres/src/btv_l1_cuda.cpp
	modules/superres/src/cuda/btv_l1_gpu.cu
	modules/superres/src/frame_source.cpp
	modules/superres/src/input_array_utility.cpp
	modules/superres/src/optical_flow.cpp
	modules/superres/src/precomp.hpp
	samples/gpu/CMakeLists.txt
	samples/gpu/brox_optical_flow.cpp
	samples/gpu/super_resolution.cpp

Conflicts:
	modules/imgproc/test/ocl/test_filters.cpp