Fixed gtkglext search in cmake.

modules/highgui/cmake/detect_gtk.cmake

@@ -29,7 +29,15 @@ if(WITH_GTK)
   if((WITH_OPENGL OR HAVE_OPENGL) AND HAVE_GTK2)
     ocv_check_modules(GTKGLEXT gtkglext-1.0)
     if(HAVE_GTKGLEXT)
-      ocv_add_external_target(gtkglext "${GTKGLEXT_INCLUDE_DIRS}" "${GTKGLEXT_LIBRARIES}" "HAVE_GTKGLEXT")
+      # HACK for https://github.com/opencv/opencv/issues/20850
+      # pkg-config reports some include directories that do not exist. Just filter them out.
+      set(GTKGLEXT_INCLUDE_DIRS_EXISTS "")
+      foreach(p ${GTKGLEXT_INCLUDE_DIRS})
+        if (EXISTS "${p}")
+          list(APPEND GTKGLEXT_INCLUDE_DIRS_EXISTS "${p}")
+        endif()
+      endforeach()
+      ocv_add_external_target(gtkglext "${GTKGLEXT_INCLUDE_DIRS_EXISTS}" "${GTKGLEXT_LIBRARIES}" "HAVE_GTKGLEXT")
     endif()
   endif()
 elseif(HAVE_GTK)