Linux now supported!

Added Makefile for linux compilation.

c2048.c:
- removed unused variables.

c2048_ai.c:
- removed unused variables.
- fixed bug in c2048_ai_destroy where it wasn't iterating over boards
  so they can be free'd.
- Fixed bug in c2048_ai_calc_monotonicity2, where I forgot braces
  around a whole block of code... whoops?! :D
  It still doesn't seem to be producing good results, so it's still
  disabled.