Specifies the prototype of log handler functions.
The default log handler, default_handler, automatically appends a new-line
character to message when printing it. It is advised that any custom log handler functions behave similarly, so that logging
calls in user code do not need modifying to add a new-line character to the message if the log handler is changed.
| log_domain |
the log domain of the message |
| message |
the message to process |
| log_level |
the log level of the message (including the fatal and recursion flags) |
| user_data |
user data, set in set_handler |