enable all compiler warnings
misc: fixed several 'set but not used' warnings, refs #33
misc: remove unused/empty function, refs #52
misc: wrap omnibot-only variable into corresponding #define
misc: remove unneccessary semicolon from end of macro
misc: fix for commit 8383f56b606c949577d1f610e5d0f5cd3a8403ce: make sure display context is initialized (src/ui/ui_shared.c:Item_OwnerDraw_Paint performs nullpointer check)