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.

Conflicts:
	CMakeLists.txt
	SConstruct

Also I squash in this commit, adding boost locale to travis:
48217f8da0a0ddf8a00fa4e93346ae467612fef4
in cmake, make PANDORA option force the BFS option to false

Conflicts:
	CMakeLists.txt

I also fix up earlier backport of this content,

cef5e2ed9ca9fcec93ae0a5f8fdf1c9fe49efb58

so that when pandora overrides BFS setting, the SDL mixer
requirement is relaxed appropriately.