suggest warning for nonexistant compare_dest

[Michael Rubel]

Hello rsync developers,

The string compare_dest (in options.c) is set whenever the --compare-dest 
or --link-dest options is called.  However, if the directory specified by 
compare_dest does not exist, then the target files (which should have been 
in compare_dest) are not found, and every transfer is a full transfer.

I can't imagine that would ever be the user's intent.  Perhaps a warning
should be added when compare_dest is non-NULL but the directory it
specifies does not exist.

This might be a common problem, because users sometimes forget that paths
are relative to the destination directory and not the caller's $PWD.  
Naturally, I made that mistake myself, and J.W. fixed it.  :)

Thanks,
Mike