Win32: Fix compilation with SDL.

`sdl-config --cflags` defines main = SDL_main
on some platforms. One of these platforms is
Windows with mingw32.

For those platforms, the solution already developed
for __APPLE__ is now applied.

A compiler warning (missing return value) is fixed, too.

Maybe __APPLE__ no longer needs a separate check.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
