FIX: Qt minimum version check fails with Qt 4.7.0

The Qt version check only passes for Qt 4.6.x. Adapted it to pass for Qt >= 4.6.0.

CMake/ctkMacroSetupQt.cmake

@@ -14,7 +14,7 @@ MACRO(ctkMacroSetupQt)
   
   IF(QT4_FOUND)
 
-    IF(NOT "${QT_VERSION_MAJOR}.${QT_VERSION_MINOR}" STREQUAL "${minimum_required_qt_version}")
+    IF("${QT_VERSION_MAJOR}.${QT_VERSION_MINOR}" VERSION_LESS "${minimum_required_qt_version}")
       MESSAGE(FATAL_ERROR "error: CTK requires Qt >= ${minimum_required_qt_version} -- you cannot use Qt ${QT_VERSION_MAJOR}.${QT_VERSION_MINOR}.${QT_VERSION_PATCH}.")
     ENDIF()