fixup mt_rng.hpp to include boost random device header correctly
fixup src/game.hpp to use boost random device header correctly
add preprocessor flags for no boost::random_device

If we don't have boost::random_device available on target
platform, then leaving the flag undefined will cause us
to use the ctime system time to get seeds.
server uses mt_rng class instead of raw boost rng

The server now holds an mt_rng for every game. Had to move mt_rng
to wesnoth_core in SConscript for this.
fixup comment
never cast random results from uint32_t to int

throughout the random wrappers and the code that uses them, make
sure to replace int's with uint32_t's.

it would be better to use boost distribution objections perhaps,
than modding rng results as we currently do.