Disable GCC warning about string truncation

Fix issue #667

I'm not sure why these warnings just started popping up (maybe the inclusion
of string in sbbs.h), but it only seems to happen in debug builds and the
possible truncation is intentional.

src/build/Common.gmake

@@ -451,7 +451,7 @@ endif
 ifdef DEBUG
  CFLAGS	+=	-ggdb
  CFLAGS	+=	-D_DEBUG
- CFLAGS +=	-Wall -Wno-char-subscripts
+ CFLAGS +=	-Wall -Wno-char-subscripts -Wno-format-truncation
 else # RELEASE
  # -finline-functions (used to) break the baja build badly.
  # This also meant that -O3 wouldn't work either.