scons: remove boostfilesystem build option, replace with "libintl"

This option cleans up some of the compatibility path code from the
1.12 release. After this commit:

- boost::filesystem becomes a hard dependency, for scons
- boost::locale is a soft dependency, a build flag "libintl"
allows to swap boost::locale out for libintl.
- when cross compiling for windows, using the "libintl" flag
results in a warning

This decision is made based on gfgtdf's comments in this forum thread:
http://forums.wesnoth.org/viewtopic.php?f=10&t=41196

It also incorporates techtonik's patch:
https://github.com/wesnoth/wesnoth/pull/347

It is left to backport the same patch for cmake, but the libintl code there
seems messed up already...