The following message is from the XZip man page:

You are expressly forbidden to use this program on an Infocom game
data file if, in so doing, you violate the copyright notice supplied
with the original Infocom game.
...
I do not know the exact copyright status of the rest, except that it
was written by Mark Howell and thus is probably copyrighted by him. He 
released it for free, so to the best of my knowledge, it can also be
distributed, modified, and used freely, with the exception noted
above.

This applies to all source code files except glkio.c, which is
copyrighted by Daniel Schepler, and glkstart.[ch], which were written
by Andrew Plotkin except for minor modifications.  The file glkio.c
may be distributed, modified, and used freely, with the exception
noted above.