Fixes for macosx 10.8

Changed -Wunused-private-field to -Wno-unused-private-field, now builds with Qt (iv)
Changed copy to copy_file to build with BOOST_FILESYSTEM_VERSION 3, in boost 1.46+

This one is up for discussion, I get this on macosx 10.8.5:

/Users/mikaelsundell/git/oiio-ms-macosx/src/libutil/filesystem.cpp:314:24: error: no member named 'copy' in namespace 'boost::filesystem'; did you mean 'copy'?
    boost::filesystem::copy (from.str(), to.str(), ec);
    ~~~~~~~~~~~~~~~~~~~^~~~
                       copy
/Users/mikaelsundell/git/oiio-ms-macosx/src/libutil/filesystem.cpp:310:13: note: 'copy' declared here
Filesystem::copy (string_view from, string_view to, std::string &err)
            ^
/Users/mikaelsundell/git/oiio-ms-macosx/src/libutil/filesystem.cpp:314:52: error: non-const lvalue reference to type 'std::string' (aka 'basic_string<char>') cannot bind to a value
      of unrelated type 'boost::system::error_code'
    boost::filesystem::copy (from.str(), to.str(), ec);
                                                   ^~
/Users/mikaelsundell/git/oiio-ms-macosx/src/libutil/filesystem.cpp:310:66: note: passing argument to parameter 'err' here
Filesystem::copy (string_view from, string_view to, std::string &err)

boost_1_47_0 - in: boost/filesystem/v3/operations.hpp

There is:
  inline
  void copy(const path& from, const path& to, system::error_code& ec) 
                                       {detail::copy(from, to, &ec);}

and...

  inline
  void copy_file(const path& from, const path& to, system::error_code& ec)
                                       {detail::copy_file(from, to, copy_option::fail_if_exists, &ec);}

Will have a closer look at this and fix the use of copy() and not copy_file()