I have compiling working on OS X Mountain Lion (10.8 ), but with a bit of messing around. For anyone curious, here are the things I had to do.
ONE: Install SDL into /Library/Frameworks as directed. I tried this numerous other ways for wanting NOT to "officially" install the framework and none worked, so I just put it there for compile time and moved on.
TWO: If you are using the Xcode Project, note that there have been several critical changes in Xcode in the past couple versions. The most notable is the disappearance of /Developer into the bundling of the self-contained "Xcode.app" now in /Applications. In 10.8, you can also no longer easily retrieve the SDKs for anything earlier than 10.6. This is a problem because BZFlag.xcodeproj wants to use libcrypto.0.9.7.dylib and libcurl.3.dylib from the 10.4 SDK. As it turns out, both are intact in the current 10.8 SDK; all you need to do is repoint Xcode to the new files.
HOW TO DO THAT
Before opening Xcode, prepare the finder. Select Xcode.app and choose "Show package contents"; then browse to Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.8.sdk/usr/
You have to do the above step because Xcode does not let you browse .app contents as you will need in the following step.
Get back to Xcode with BZFlag.xcodeproj opened. Make sure you have the left-pane on and are looking at the file browser. Expand/disclose "Frameworks & Libraries" and you should see the missing dylib files highlighted with red text; select one. Now make sure you have the right-pane (Utilities view) enabled. You should see "Location" under the "Identity" area and there should be two icons, a dialog box and an arrow. Click the dialog box and Xcode prompts you to choose a new file. With this dialog open, switch back to Finder where you have the 10.8 SDK open. Drag the "lib" folder from the 10.8 SDK into the Xcode selection dialog box. Now find the correct library (libcrypto.0.9.7.dylib and libcurl.3.dylib)
THREE: Update the deployment target to 10.8. In the left-pane of Xcode, make sure you are in file browser mode and choose the project at the top of the hierarchy. The center pane of Xcode should now have some options at the top, including "Summary". Select "Summary" and you should see "Deployment Target" near the top. It's a drop-down menu and you should probably just choose 10.8, though I did get a successful build in 10.6 and 10.7
FOUR: Using --enable-debug in the configure script causes warnings to be treated as errors. You will get a lot of warnings related to using a C99 hexadecimal floating constant. Select the Project like you did in above, but now choose "Build Phases" to edit the build steps. The second phase is "Run Script" and includes the configure call; remove the --enable-debug option to forget these warnings.
If you want to address the warnings regarding the C99 hexadecimal floating constant, see
http://ibot.rikers.org/%23bzflag/20120729.html.gz. You will find notes that you can edit include/common.h and preface the "#ifndef MAXFLOAT" with "#undef MAXFLOAT" -- this worked for me, as well.
FIVE: During a couple of my earlier builds, I found that while configure detected the SDL framework, it was not finding SDL/SDL.h -- odd! At one point, I added the --with-SDL option to configure and voila! that run (and all going forward) then saw SDL/SDL.h. I don't know if this was coincidence or successful troubleshooting, but it's worth noting.
Before SDL/SDL.h was seen, the build would always stop, of course, at the SDL-related code due to tons of undefined procedures and/or objects. I should also mention that when attempting NOT to use SDL, the MacMedia etc files complained of similar errors. In particular, if all else succeeds MacDisplay.cxx fails because it refers to a NewRgn() call that I could not find defined anywhere else in the entire project... admittedly a quick search using only grep.
FINALLY, I should note that in some builds, Xcode complained of invocation errors in the shell scripts, but that the script/shell did not exit with failure status. Whatever caused this did not seem to affect the build results, so I haven't worried about it.
I hope this helps other folks get moving.