CMake's logging is restricted to using the built in message() function. It writes the messages to stdout and stderr depending on the given tag present (STATUS, ERROR, FATAL_ERROR,WARNING, <none>). This is sometimes not enough - especially when the output of your CMake script should be very controlled (ie. it is important that no debug or status messages are ouput when users expect the output to adher to a certain format)
This is why I started to write log functions which do not output anything. You can listen to log messages using the event system - the on_log_message is called for every log message that is output.