Ok so this is really weird.. Was making sure all my Xcode stuff was up to date, and I had all the command line tools installed, etc.. Clang itself is giving the error. I just tried to run clang on its own to check the version and got this:
SHIVA:mame mothra$ clang
dyld: Symbol not found: __ZNKSt3_V214error_category10_M_messageB5cxx11Ei
Referenced from: /usr/local/bin/clang
Expected in: /usr/lib/libstdc++.6.0.9.dylib
Abort trap: 6
Yet Ive built other stuff since I first attempted MAME? Just built the latest svn of QMC2 about a minute ago with no trouble (but then again it looks like its using g++ and not clang to build).