Set CMAKE_CXX_STANDARD in enable_latest_cxx_support only if cmake version supports it.

cmake/enable_latest_cxx_support.cmake

@@ -30,14 +30,19 @@ if(COMPILER_SUPPORTS_CXX17)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++17")
     set(${PROJECT_NAME}_CXX_FLAGS "${${PROJECT_NAME}_CXX_FLAGS} -std=c++17")
     set(${PROJECT_NAME}_CMAKE_CXX_FLAGS "${${PROJECT_NAME}_CMAKE_CXX_FLAGS} -std=c++17")
+    if(NOT ${CMAKE_VERSION} VERSION_LESS "3.8")
+      set(CMAKE_CXX_STANDARD 17)
+    endif()    
 elseif(COMPILER_SUPPORTS_CXX14)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14")
     set(${PROJECT_NAME}_CXX_FLAGS "${${PROJECT_NAME}_CXX_FLAGS} -std=c++14")
     set(${PROJECT_NAME}_CMAKE_CXX_FLAGS "${${PROJECT_NAME}_CMAKE_CXX_FLAGS} -std=c++14")
+    set(CMAKE_CXX_STANDARD 14)    
 elseif(COMPILER_SUPPORTS_CXX11)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
     set(${PROJECT_NAME}_CXX_FLAGS "${${PROJECT_NAME}_CXX_FLAGS} -std=c++11")
     set(${PROJECT_NAME}_CMAKE_CXX_FLAGS "${${PROJECT_NAME}_CMAKE_CXX_FLAGS} -std=c++11")
+    set(CMAKE_CXX_STANDARD 11)
 elseif(COMPILER_SUPPORTS_CXX0X)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x")
     set(${PROJECT_NAME}_CXX_FLAGS "${${PROJECT_NAME}_CXX_FLAGS} -std=c++0x")