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You should install Apple's XCode to get the compilers required for ScummVM.
Also, ScummVM needs several libraries, foremost SDL. An easy way to install those is using Fink or MacPorts. Alternatively, you can install these manually. See Compiling_ScummVM/Mac_OS_X_10.2.8 for some (possibly outdated) notes on that.
TODO: List which packages should be installed, ideally give instructions for both Fink and MacPorts