Update gettext_boost.cpp

backports
https://github.com/wesnoth/wesnoth/commit/ae86186db772003cca62d2af6ed9553839af0f0d
https://github.com/wesnoth/wesnoth/commit/c96db7a17e53e7e0ee0bde393e5b10797bab5355
https://github.com/wesnoth/wesnoth/commit/e99202c580354af6d1995d5fb8fdcd4d3c01e011