Require cmake 3.9 if Qt5 is used

CMakeLists.txt

@@ -16,6 +16,10 @@ armarx_project(ArmarXGui)
 OPTION(ARMARX_USE_QT5 "if on, uses qt5. uses qt4 otherwise" OFF)
 export_variable_to_package_config(ARMARX_USE_QT5)
 
+if(ARMARX_USE_QT5 AND NOT "${CMAKE_VERSION}" VERSION_GREATER 3.9)
+    message(FATAL_ERROR "If using Qt5 the cmake version must be above 3.9 (current version = ${CMAKE_VERSION})")
+endif()
+
 add_subdirectory(etc)
 add_subdirectory(source)