mprove Gtk2/3 options in cmake

Update to cmake files for to include minimum versions, and tidy up gtk operation.

Files updated:
CMakeLists.txt:
  WITH_GTK now uses Gtk3 by default. If not found then Gtk2 is used.
  WITH_GTK_2_X forces Gtk2.x use

cmake/OpenCVFindLibsGUI.cmake
  Updated selection logic to implement methodology described above.
  Implemented warning if Gtk3 not found (and not overridden)
  Implemented error if Gtk does not meet minimum required version

cmake/OpenCVMinDepVersions.cmake
  Added minimum Gtk version of 2.18.0

CMakeLists.txt

@@ -120,7 +120,7 @@ OCV_OPTION(WITH_FFMPEG         "Include FFMPEG support"                      ON
 OCV_OPTION(WITH_GSTREAMER      "Include Gstreamer support"                   ON   IF (UNIX AND NOT APPLE AND NOT ANDROID) )
 OCV_OPTION(WITH_GSTREAMER_1_X  "Include Gstreamer 1.x support"               OFF)
 OCV_OPTION(WITH_GTK            "Include GTK support"                         ON   IF (UNIX AND NOT APPLE AND NOT ANDROID) )
-OCV_OPTION(WITH_GTK3           "Include GTK3 support"                        ON   IF (UNIX AND WITH_GTK AND NOT APPLE AND NOT ANDROID) )
+OCV_OPTION(WITH_GTK_2_X        "Use GTK version 2"                           OFF  IF (UNIX AND NOT APPLE AND NOT ANDROID) )
 OCV_OPTION(WITH_IPP            "Include Intel IPP support"                   OFF  IF (MSVC OR X86 OR X86_64) )
 OCV_OPTION(WITH_JASPER         "Include JPEG2K support"                      ON   IF (NOT IOS) )
 OCV_OPTION(WITH_JPEG           "Include JPEG support"                        ON)

cmake/OpenCVFindLibsGUI.cmake

@@ -41,12 +41,21 @@ endif()
 # --- GTK ---
 ocv_clear_vars(HAVE_GTK HAVE_GTK3 HAVE_GTHREAD HAVE_GTKGLEXT)
 if(WITH_GTK AND NOT HAVE_QT)
-  if(WITH_GTK3)
-    CHECK_MODULE(gtk+-3.0 HAVE_GTK3)
-  else()
+  if(WITH_GTK_2_X)
     CHECK_MODULE(gtk+-2.0 HAVE_GTK)
+    if(HAVE_GTK AND (ALIASOF_gtk+-2.0_VERSION VERSION_LESS MIN_VER_GTK))
+      message (FATAL_ERROR "Gtk support requires a minimum gtk+ version of ${MIN_VER_GTK} (${ALIASOF_gtk+-2.0_VERSION} found)")
+    endif()
+  else()
+    CHECK_MODULE(gtk+-3.0 HAVE_GTK3)
+    if(NOT HAVE_GTK3)
+      message(WARNING "Unable to locate Gtk3 development libraries")
+    endif()
   endif()
   CHECK_MODULE(gthread-2.0 HAVE_GTHREAD)
+  if(HAVE_GTK OR HAVE_GTK3 AND NOT HAVE_GTHREAD)
+    message(FATAL_ERROR "gthread not found. This library is required when building with Gtk support")
+  endif()
   if(WITH_OPENGL AND NOT HAVE_GTK3)
     CHECK_MODULE(gtkglext-1.0 HAVE_GTKGLEXT)
   endif()

cmake/OpenCVMinDepVersions.cmake

 set(MIN_VER_CMAKE 2.8.7)
 set(MIN_VER_PYTHON 2.6)
 set(MIN_VER_ZLIB 1.2.3)
+set(MIN_VER_GTK 2.18.0)