The MacOSX target of Squeak VM currently uses some old Mac Carbon API.
This might be a problem with a recent version of Xcode, none of which is distributed with such backward compatibility support.
However, there is at least one solution: using the shell script provided at https://github.com/devernay/xcodelegacy worked like a charm for me (on OSX 10.10.4, with Xcode 6.4).
I can even compile my on brand of squeak.cog.spur (VMMaker.oscog-nice.1428 from http://smalltalkhub.com/mc/nice/NiceVMExperiments/main) with Apple LLVM 6.1.0 (clang-602.0.53) and get no regression against Eliot's r3410 when running the trunk tests.
That's allways good to know.