This seems to happen to me a lot, but I've just tried rebuilding from current svn trunk (r230645) and it fails:
[ 15%] Building CXX object projects/compiler-rt/lib/sanitizer_common/CMakeFiles/RTSanitizerCommon.iossim.dir/sanitizer_procmaps_linux.cc.o
ld: requested re-export symbol std::set_unexpected(void (*)()) is not from a dylib, but from CMakeFiles/cxx.dir/__/src/exception.cpp.o
for architecture x86_64
This is on OS X 10.8.5, building with Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn) (from Xcode 5.0.2), and CMake 3.2rc2.