Bugfix: allow additional_features to be empty

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@217268 91177308-0d34-0410-b5e6-96231b3b80d8
Allow libc++ to be built on systems without POSIX threads

If you're crazy enough to want this sort of thing, then add
-D_LIBCPP_HAS_NO_THREADS to your CXXFLAGS and
--param=additiona_features=libcpp-has-no-threads to your lit commnad line.

http://reviews.llvm.org/D3969


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@217271 91177308-0d34-0410-b5e6-96231b3b80d8