I'm unsure what this include is for. Is seems a bit odd to have an include in __config which currently seems to purely be a 『compiler define -> libc++ configuration define』 mapping.

Even if we kept it here, I think we need to wrap it in an !_WIN32 like is done here https://github.com/awong-dev/ndk/blob/enable_single_threaded_libcxx/sources/cxx-stl/llvm-libc%2B%2B/libcxx/src/thread.cpp#L21

However, I don't think unistd.h should be in __config...