An Undefined reference to decgraphics_mode_callback was possible
if built for tty only.
drawing.c had an #include "tcap.h" which is what actually defined
TERMLIB. It isn't needed in drawing.c anymore, but it is needed
in symbols.c, in order to get the define for TERMLIB so that
decgraphics_mode_callback variable gets defined.
The undefined reference was from win/tty/termcap.h in code that
was #ifdef TERMLIB, but win/tty/termcap.h has the #include "tcap.h"
