ENH: clarify Qt4 superbuild warning message

SuperBuild/CMake/External_otb.cmake

@@ -65,9 +65,6 @@ endif()
 
 if(OTB_USE_QT4)
   list(APPEND ${proj}_DEPENDENCIES QT4)
-  if(USE_SYSTEM_QT4 AND UNIX)
-    message(STATUS "  Qt4 is not built by SuperBuild. You need to install it via package manager.")
-  endif()
 endif()
 
 INCLUDE_SUPERBUILD_DEPENDENCIES(${${proj}_DEPENDENCIES})

SuperBuild/CMake/External_qt4.cmake

@@ -12,6 +12,9 @@ if(USE_SYSTEM_QT4)
 else()
   SETUP_SUPERBUILD(PROJECT ${proj})
   message(STATUS "  Using Qt4 SuperBuild version")
+  if(UNIX)
+    message(STATUS "  SuperBuild may fail to compile Qt4. If so, you should install it via package manager.")
+  endif()
   
   if(NOT DEFINED git_protocol)
     set(git_protocol "git")