Don't delete copy constructor when rvalue references are disabled

Before SVN r82706 from February 2013 (git a7091d1009f057b1148b36fabf91f83667db5f72), Boost.Move worked perfectly for compilers that supported deleted functions without supporting rvalue references (for example, clang with libstdc++ 4.2.1 on Mac to support OS X 10.6). r82706 broke those setups as temporaries now call the deleted copy constructor instead of the move constructor. This change reverts those setups to the old behavior.