Ai boost filesystem

AI's the boost filesystem branch contains 2 parts:
1) replace our filesystem.cpp with a boost based version (filesystem_boost.cpp) which supports utf8 on all systems. (that's in this pr)
2) make SDL use our filesystem functions (that's not in this pr)
in order to gain utf8 support on window we need both.


the filesystem_boost.cpp file replaces the filesystem.cpp but just like in ai's original branch this still leaves the original filesystem.cpp file so one can choose whether to compile filesystem.cpp or filesystem_boost.cpp. Thats also why i have less concerns abouth merging this untested (at lest not very tested by me) branch.

Also note that this doesnt contain updates to any other build mechanism than scons so and other build sytem will still use filesystem.cpp unless you change it manually.