toh: Add IS_LEGAL_MOVE() macro
toh: Add game function prototypes
toh: Define mprintf() macro

Replace inline message function with a macro which rids the need for the
user of vprintf.
toh: Set backend with makefile variable

It is no longer necessary to manually -D define the backend variable at
compile time, instead, set the BACKEND variable when invoking make.
toh: Add IS_EVEN() macro
toh: Include gertboard header first.
toh: Add an autonomous backend

This backend implements an unsophisticated quasi-brute force algorithm
for completing a game with 100% efficiency.
Merge branch 'wip/toh'
Merge branch 'wip/toh-autonomous'