Is it safe to use the Windows "Compress directory to save space" feature on the directory that contains my MySQL general log file?
The Windows feature to compress a directory is transparent to all applications.  However, it does slow down I/O operations some.
The General Log should not be left on forever; it is a big disk hog.  Instead, turn it on only long enough to get the necessary debugging info, then turn it off and toss the log file.
Or, you could log to TABLE and not FILE; I think the table has a limit.  (Check the settings.)