Fixup boost filesystem options for cmake, scons

The option should control the compilation, in wesnoth-core, of
filesystem_boost.cpp, and gettext_boost.cpp, vs. the old versions.

Using the new code is now the default option, you must explicitly
deactivate with both cmake and scons to get the old stuff back.
add boost locale to travis
in cmake, make PANDORA option force the BFS option to false
for cmake + ENABLE_PANDORA, reduce SDL_mixer dep to 1.2 from 1.2.12
use LOG_G instead of std::cerr
move some savegame manipulation to saved_game.cpp
fix saved_game::cancel_orders()
fix start-of-scenario 1.13 compability (#22546)