CMake: check that ROOT has C++17

CMakeLists.txt

@@ -148,10 +148,8 @@ IF(ROOT_CXX_FLAGS MATCHES ".*std=c\\+\\+1[7z].*")
     IF(NOT SUPPORT_STD_CXX17)
         MESSAGE(FATAL_ERROR "ROOT was built with C++17 support but current compiler doesn't support it")
     ENDIF()
-ELSEIF(ROOT_CXX_FLAGS MATCHES ".*std=c\\+\\+1[14y].*")
-    SET(CMAKE_CXX_STANDARD 14)
 ELSEIF(ROOT_CXX_FLAGS MATCHES ".*std=c\\+\\+.*")
-    MESSAGE(FATAL_ERROR "ROOT was built with an unsupported C++ version: ${ROOT_CXX_FLAGS}")
+    MESSAGE(FATAL_ERROR "ROOT was built with an unsupported C++ version, C++17 is required: ${ROOT_CXX_FLAGS}")
 ELSE()
     MESSAGE(FATAL_ERROR "Could not deduce ROOT's C++ version from build flags: ${ROOT_CXX_FLAGS}")
 ENDIF()