Update 5/8/2011 I have received word that this tutorial works with Xcode 3.2.4 as well as Xcode 4. Thanks to andreb. I'd like to thank him for pointing out I could use the $SRCROOT variable instead of /absolute/path/to/workspace/ZDoom-Xcode.
When I was younger, I used to make maps and other mods for the popular old school game Doom. The community of mappers for the game was already quite established by the time I got into it, due to Doom being one of the original games to develop a large modding community. Even after it's heyday (and still to this day) the community thrives due to two things: the game is classically enjoyable and fun to mod for and the existence of source ports has kept the game relatively up to date. Almost no one runs vanilla Doom these days, due to the slow but steady deprecation of DOS support on many systems but plenty of programmers have kept the game alive by creating modified engines (or source ports) which run on modern operating systems without a hitch and add features to the game to make it more interesting while allowing it to keep it's charm and simplicity. This is thanks to John Carmack and id Software's open sourcing of the Doom engine back in 1997. ZDoom is one of the more popular updated engines and has been around for a long time.
A few years ago, I switched from PC to Mac and although I am happy with this change the one thing I left behind was gaming. Although I don't care too much these days if the latest game is being ported to OS X or not, I did miss some of the old games I used to play. There are so many free maps and add ons from the Doom community that just buying Doom, downloading a source port and getting some mods is enough to keep one entertained indefinitely. It will also only set you back about ten dollars to purchase Doom, making it extremely cheap gaming.
There are plenty of ports for Doom which release OS X binaries and these are all great ports, however ZDoom has such a large number of maps and features that you miss out on a lot of good mods if you aren't running it. I had never gotten around to running it on OS X before because they didn't have any binaries, but I finally decided to dive in and build it myself. Luckily, they support OS X by using CMake, but unluckily, nothing works perfectly and this is what had halted me from getting too deep before. As great as running CMake and then make would be, the build process doesn't work quite that easily. Before I continue however, I discovered that the DRD Team already hosted unofficial OS X builds of ZDoom so if you would like to spare yourself a headache and just download those, you can run ZDoom on OS X without building anything.