Do not assume a specific Qt5 install structure.

CMake/ctkMacroSetupQt.cmake

@@ -33,10 +33,7 @@ macro(ctkMacroSetupQt)
   if(CTK_QT_VERSION VERSION_GREATER "4")
     cmake_minimum_required(VERSION 2.8.12)
     set(QT5_INSTALL_PREFIX "" CACHE PATH "The install location of Qt5")
-    if(NOT QT5_INSTALL_PREFIX OR NOT EXISTS ${QT5_INSTALL_PREFIX}/bin/qmake)
-      message(FATAL_ERROR "You must specify the install location of Qt5")
-    endif()
-    set(CMAKE_PREFIX_PATH ${QT5_INSTALL_PREFIX})
+    set(CMAKE_PREFIX_PATH ${CMAKE_PREFIX_PATH} ${QT5_INSTALL_PREFIX})
     set(CTK_QT5_COMPONENTS Core Xml XmlPatterns Concurrent Sql Test)
     if(CTK_LIB_Widgets OR CTK_LIB_CommandLineModules/Frontend/QtGui)
       list(APPEND CTK_QT5_COMPONENTS Widgets OpenGL UiTools)