- Adjusted the dll-exported functions (calling convention / _cdecl
  symbol decoration mess), so that the applications linked against
  mingw- or msvc-built SDL.dll can run with the watcom-built dll.
- Builds current SDL-1.2 branch, tested by cross-compiling on Linux.
- The makefile builds the dll by default, and links statically to
  watcom C libraries instead of their dll versions (dropped '-br').
- Included the needed directx headers (ddraw.h, dinput.h, dsound.h)
  based on Wine project (LGPL). (Can still override using DXDIR=xx
  on the wmake cmdline..)