-DBOOST_ASIO_HAS_STD_STRING_VIEW=1 fixes:
/monero/contrib/depends/x86_64-unknown-freebsd/native/usr/include/c++/v1/experimental/string_view:18:3:
warning: "<experimental/string_view> has been removed. Use <string_view> instead." [-W#warnings]
/monero/contrib/depends/x86_64-unknown-freebsd/include/boost/asio/detail/string_view.hpp:32:12
: error: no member named 'experimental' in namespace 'std'
using std::experimental::basic_string_view;
~~~~~^