Add initial support for the SDL 2.0. Still WiP, there are some
issues with background being erased by the foreground and palettes
are not quite working, but the control works and video mode being
setup correctly.
Another instance where SDLK_XX needs to be converted to the SDL_SCANCODE_XX.
Convert icon from the indexed color representation into 32-bit
RGBA array. This simplifies initialization.
It's unclear why did I put "doscreenupdate()" into the kbhit routine,
it certainly does not belong there.
o Get rid of unused rootpal;

o implement fullscreen mode;

o implement window icon.

This makes SDL 2.0 pretty much on par with SDL 1.2 version, if
not better.
Make this working with SDL 2.0 properly. The main difference is that
we need to explicitly silence-out the buffer in the callback. Dending
on the format, it means filling it with some non-zero data, 127 for U8
in particular.
Add Pi definition.
Filter out sound via first order low pass recursive filter with
the cutoff frequency of 5kHz.
Add prototype for the doscreenupdate() function.w
In the "flashywait" function, run doscreenupdate() function, since
SDL 2.0 now implements "software" palettes, so in order for the palette
change to have an effect we need to run that.
Decrease filter cut-off frequency to 4kHz.
o Add missed <stdio.h>;

o conditionalize sdl_set_x11_parent on ifdef UNIX.
Include <stdio.h> when DIGGER_DEBUG is set.
Add missed <math.h> and <stdio.h>.
Include <stdio.h> when DIGGER_DEBUG is defined.
Fix sound on Windows/SDL.
Fix cross-compile on MinGW.
There is no "win_xxx.h" headers in this package, so that don't
try to unclude them.
GC lot of unused defines and conditionals for old platforms:

o _WINDOWS
o __WIN32__
o WIN16
o GNU32
o FLATFILE
o DIRECTX
o RUNTIMEDYNAMICLINK

This makes code much more clean and maintenable. Please note that
it does not mean Windows is unsupported now, quite on contrary,
SDL 2.0 version for Windows works great and we will be publishing
some pre-built binaries for that platform.
Add Windows icon and simple resource RC file.