CMake: allow legacy C++ standard names

(cherry picked from commit e9d5cc08)

CMakeLists.txt

@@ -129,11 +129,11 @@ IF(NOT ROOT_FOUND)
 ENDIF()
 
 # Downgrade to C++14 if ROOT is not build with C++17 support
-IF(ROOT_CXX_FLAGS MATCHES ".*std=c\\+\\+17.*")
+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[14].*")
+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}")