cmake HAVE_SYSTEM_MYSQL examples

rAthenaCN/rAthenaCN 3rdparty/mysql/CMakeLists.txt :65 

mark_as_advanced( HAVE_SYSTEM_MYSQL )

rAthenaCN/rAthenaCN 3rdparty/mysql/CMakeLists.txt :67 

unset( HAVE_SYSTEM_MYSQL CACHE )