Qt-Qwt dependency in superbuild

This is a small MR to warn user that are disabling Qt but not Qwt that the Qt module will still be build as QWT depends on it.
As it just produces a message we can discuss on whether we are creating a depending option for Qwt or we force the value of OTB_USE_QT to ON if OTB_USE_QWT=ON.

SuperBuild/CMakeLists.txt

@@ -329,7 +329,8 @@ else()
 endif()
 
 option(OTB_USE_SHARK "Enable module Shark in OTB" ON)
-option(OTB_USE_QWT "Enable module QWT in OTB" ON)
+include (CMakeDependentOption)
+CMAKE_DEPENDENT_OPTION( OTB_USE_QWT "Enable module QWT in OTB" ON "OTB_USE_QT" OFF)
 option(OTB_USE_GSL "Enable module GSL in OTB" ON)
 
 option(OTB_USE_MPI "Enable MPI in OTB" OFF)