change setting cxx standard

CMakeLists.txt

@@ -25,7 +25,7 @@ include(CheckCXXCompilerFlag)
 CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)
 if(COMPILER_SUPPORTS_CXX11)
   message(STATUS "The compiler ${CMAKE_CXX_COMPILER} has C++11 support.")
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+  set(CMAKE_CXX_STANDARD 11)
 else()
   message(STATUS "The compiler ${CMAKE_CXX_COMPILER} has no C++11 support. Please use a different C++ compiler.")
 endif()