Set -D_LIBCPP_HAS_NO_THREADS and -D_LIBCPP_HAS_NO_MONOTONIC_CLOCK based on available_features

http://reviews.llvm.org/D5214


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@217261 91177308-0d34-0410-b5e6-96231b3b80d8