Installing libcxx (make install) assumes root privileges

When installing libcxx via `make install`, the destination directory
receives root privileges and causes the installation to fail:

    [..]
    chown -R root:wheel PREFIX/lib/c++
    chown: PREFIX/lib/c++/v1/__bit_reference: Operation not permitted
    chown: PREFIX/lib/c++/v1/__config: Operation not permitted
    chown: PREFIX/lib/c++/v1/__debug: Operation not permitted
    chown: PREFIX/lib/c++/v1/__functional_03: Operation not permitted
    chown: PREFIX/lib/c++/v1/__functional_base: Operation not permitted
    chown: PREFIX/lib/c++/v1/__functional_base_03: Operation not permitted
    [..]

It works obviously with `sudo make install`, but I wonder why user
installations are not supported!?

    Matthias