fix hardcoded cpp standard

CMakeLists.txt

@@ -35,7 +35,9 @@ if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
   set (CMAKE_CXX_FLAGS "--std=gnu++11 ${CMAKE_CXX_FLAGS}")
 endif ()
 else ()
-  set (CMAKE_CXX_STANDARD 11)
+  if(NOT DEFINED CMAKE_CXX_STANDARD)
+    set(CMAKE_CXX_STANDARD 11)
+  endif()
 endif ()
 
 list(APPEND CMAKE_PREFIX_PATH $ENV{ROOTSYS} ${ROOTSYS})