[libc++] Fix the CMake build on Mac when setting MACOSX_DEPLOYMENT_TARGET=10.6

The build file was trying to use LIBCXX_VERSION, which isn't set
anywhere, and also forgot to split the 'compile_flags' list.

Differential Revision: http://reviews.llvm.org/D4860

llvm-svn: 215463
1 file changed