boost/random/detail/polynomial.hpp:303:20: -Wdeprecated-copy warning

```
clang-linux.compile.c++ ../../../../bin.v2/libs/spirit/test/qi/qi_range_run.test/clang-linux-12/release/cxxstd-11-iso/stdlib-libc++/threading-multi/visibility-hidden/range_run.o
In file included from range_run.cpp:17:
In file included from ../../../../boost/random/mersenne_twister.hpp:32:
../../../../boost/random/detail/polynomial.hpp:303:20: error: definition of implicit copy constructor for 'reference' is deprecated because it has a user-declared copy assignment operator [-Werror,-Wdeprecated-copy]
reference &operator=(const reference &other)
^
../../../../boost/random/detail/polynomial.hpp:315:16: note: in implicit copy constructor for 'boost::random::detail::polynomial::reference' first required here
return reference(_storage[i/bits], i%bits);
^
```