games/wesnoth: unbreak with boost 1.69

src/units/frame.cpp:463:53: error: no viable conversion from 'const boost::tribool' to 'bool'

v.emplace_back("auto_vflip=" + utils::bool_string(auto_vflip_));
                                                  ^~~~~~~~~~~

src/serialization/string_utils.hpp:282:36: note: passing argument to parameter 'value' here
std::string bool_string(const bool value);

^

src/units/frame.cpp:467:53: error: no viable conversion from 'const boost::tribool' to 'bool'

v.emplace_back("auto_hflip=" + utils::bool_string(auto_hflip_));
                                                  ^~~~~~~~~~~

src/serialization/string_utils.hpp:282:36: note: passing argument to parameter 'value' here
std::string bool_string(const bool value);

^

src/units/frame.cpp:471:56: error: no viable conversion from 'const boost::tribool' to 'bool'

v.emplace_back("primary_frame=" + utils::bool_string(primary_frame_));
                                                     ^~~~~~~~~~~~~~

src/serialization/string_utils.hpp:282:36: note: passing argument to parameter 'value' here
std::string bool_string(const bool value);

^

src/units/frame.cpp:775:13: error: no viable conversion from 'boost::logic::tribool' to 'const bool'

const bool primary = result.primary_frame == true || boost::logic::indeterminate(result.primary_frame);
           ^         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

PR: 232525