There's a bit of a glitch in the XCode project of MacMame. What happens is that Mac OS X can have two different file systems: a case-sensitive one, and a case-insensitive one.
Since I assume whoever made the XCode project file has the case-INsensitive filesystem, the XCode project contains references to source files that are called, say, "I86.c", which of course refers to the source file "i86.c". In the case-insensitive filesystem the two names are the same, but not so in the case-sensitive filesystem.
Of course, to fix this one just needs to rename the source file references in the XCode project, but I thought I'd bring this up since it would save some hassle to the would-be contributor.