Howto: Compress Broken Sword

I just tried to compress the data from my CD of Broken Sword to use it with scummvm, but I stumbled over an issue that made me mad.
$ scummvm-tools-cli --tool compress_sword1 --flac --best .

Unable to find speech files.
Please copy the SPEECH.CLU files from Broken Sword CD1 and CD2
into the game directory on your disk or into a "SPEECH" subdirectory
and rename them to SPEECH1.CLU and SPEECH2.CLU

If your OS is case-sensitive, make sure the filenames
and directorynames are all upper-case.
But I had done everything that was obvious: The files hat the right name (even in uppercase) and the right location, but I still got that message. So finally I debugged the compress_sword1.cpp and noticed that the tool was not using my full path. The solution was to use a trailing slash in the path (e.g.: ./) or even append a filename (e.g.: ./SPEECH1.CLU). So the tool seems to expect a file-name, but then works on all files in that directory. Very strange behavior.