Added a new type in board.h: GamePositionX and the related functions ...
The works is not completed but everything works and tests are already done.

Added:
 * game_position_x_new
 * game_position_x_free
 * game_position_x_empties
 * game_position_x_get_player
 * game_position_x_get_opponent
 * game_position_x_legal_moves

Still pending:
 * game_position_x_count_difference
 * game_position_x_to_string
 * game_position_x_print
 * game_position_x_compare
 * game_position_x_clone
 * game_position_x_has_any_legal_move
 * game_position_x_has_any_player_any_legal_move
 * game_position_x_is_move_legal
 * game_position_x_make_move
 * game_position_x_pass
 * game_position_x_hash
 * game_position_x_final_value