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

This option cleans up some of the compatability 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