GCC suggest-override warnings

Start by reproducing the GCC 7.5 build with -Werror=suggest-override, then inspect the functions reported in bad_weak_ptr.hpp, local_counted_base.hpp, and sp_counted_impl.hpp. Compare the reported methods with their base declarations and run the smart_ptr test suite. Done means the listed warnings no longer fail the build without breaking supported compiler compatibility.

GCC 7.5 fires suggest-override warnings in Boost 1.72:

include/boost/smart_ptr/bad_weak_ptr.hpp:50:26: error: ‘virtual const char* boost::bad_weak_ptr::what() const’ can be marked override [-Werror=suggest-override]
include/boost/smart_ptr/detail/local_counted_base.hpp:116:18: error: ‘virtual void boost::detail::local_counted_impl::local_cb_destroy()’ can be marked override [-Werror=suggest-override]
include/boost/smart_ptr/detail/local_counted_base.hpp:121:41: error: ‘virtual boost::detail::shared_count boost::detail::local_counted_impl::local_cb_get_shared_count() const’ can be marked override [-Werror=suggest-override]
include/boost/smart_ptr/detail/local_counted_base.hpp:133:18: error: ‘virtual void boost::detail::local_counted_impl_em::local_cb_destroy()’ can be marked override [-Werror=suggest-override]
include/boost/smart_ptr/detail/local_counted_base.hpp:138:41: error: ‘virtual boost::detail::shared_count boost::detail::local_counted_impl_em::local_cb_get_shared_count() const’ can be marked override [-Werror=suggest-override]
include/boost/smart_ptr/detail/sp_counted_impl.hpp:88:18: error: ‘void boost::detail::sp_counted_impl_p::dispose() [with X = boost::iostreams::symmetric_filter >, std::allocator >::impl]’ can be marked override [-Werror=suggest-override]
include/boost/smart_ptr/detail/sp_counted_impl.hpp:96:20: error: ‘void* boost::detail::sp_counted_impl_p::get_deleter(const sp_typeinfo_&) [with X = boost::iostreams::symmetric_filter >, std::allocator >::impl; boost::detail::sp_typeinfo_ = std::type_info]’ can be marked override [-Werror=suggest-override]
include/boost/smart_ptr/detail/sp_counted_impl.hpp:101:20: error: ‘void* boost::detail::sp_counted_impl_p::get_local_deleter(const sp_typeinfo_&) [with X = boost::iostreams::symmetric_filter >, std::allocator >::impl; boost::detail::sp_typeinfo_ = std::type_info]’ can be marked override [-Werror=suggest-override]
include/boost/smart_ptr/detail/sp_counted_impl.hpp:106:20: error: ‘void* boost::detail::sp_counted_impl_p::get_untyped_deleter() [with X = boost::iostreams::symmetric_filter >, std::allocator >::impl]’ can be marked override [-Werror=suggest-override]