Fix the CMake build on Mac when setting MACOSX_DEPLOYMENT_TARGET=10.6

The build file was trying to use LIBCXXABI_VERSION, which isn't set
anywhere.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@215465 91177308-0d34-0410-b5e6-96231b3b80d8