The sources should just compile. Tested with GCC 1.40 & GCC 5.10. I've included some games to test from here:
http://www.ifarchive.org/indexes/if-archiveXgamesXzcode.html
advent.z3 [03-Oct-2015]
Adventure aka Colossal Cave, the original 350 points version ported to ZIL by Jesse McGrew, based on ealier work by Graham Nelson and Dave Baggett, with some elements of the original Fortran version restored. Release 1 / Serial number 151001 (source code is in infocom/compilers/zilf/zilf-0.7.zip)
catseye.z3 [24-Oct-2007]
Cat's Eye, Miniventure #2 by Dave Bernazzani, release 3. "When your uncle Xevion asks you for a favor, you simply can't refuse. But what mysteries lie in his strange house?"
curses.z3 [26-Nov-2002]
Curses, An Interactive Diversion by Graham Nelson. Release 7 / Serial number 930428 Note: This was the first version of Curses to appear in the archive, and is kept here for historical interest. If you want to play Curses, you should get curses.z5 instead.
dejavu.z3 [14-May-1997]
Deja Vu, An Interactive Demonstration by Graham Nelson Release 1 / Serial Number 930921 Note: This game is archived here only for its historical interest, being the earliest inform-compiled file known to exist. Do not pester Graham about it!
gussdeath.z3 [31-Jan-2004]
Guss's Death, an Exercise in Interactive Fiction, by Kyle Frownfelter. Release 1 / Serial number 040131
moonglow.z3 [24-Oct-2007]
Moonglow, Miniventure #1 by Dave Bernazzani, release 3. "What do you do when a mysterious orb crash-lands in your field? It's not your typical day at the farm!"
Pretty much just run:
gcc *.c -o info.exe
info advent.z3
and you'll be in business.