Improve GTK+ library selection logic.

The new logic will select GTK+3 by default if WITH_GTK is selected. If
the GTK+3 libraries are not found, then GTK+2 libraries will be selected
if found.
This can be overridden by using WITH_GTK_2_X to force selection of GTK+2
(if found).

cmake/OpenCVFindLibsGUI.cmake

@@ -41,18 +41,14 @@ endif()
 # --- GTK ---
 ocv_clear_vars(HAVE_GTK HAVE_GTK3 HAVE_GTHREAD HAVE_GTKGLEXT)
 if(WITH_GTK AND NOT HAVE_QT)
-  if(WITH_GTK_2_X)
+  if(NOT WITH_GTK_2_X)
+    CHECK_MODULE(gtk+-3.0 HAVE_GTK3)
+    set(HAVE_GTK TRUE)
+  elseif(NOT HAVE_GTK3)
     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 version of ${MIN_VER_GTK} (${ALIASOF_gtk+-2.0_VERSION} found)")
     endif()
-  else()
-    CHECK_MODULE(gtk+-3.0 HAVE_GTK3)
-    if(HAVE_GTK3)
-      set(HAVE_GTK ON)
-    else()
-      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)