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

vl.c

@@ -113,12 +113,12 @@
 #endif
 
 #ifdef CONFIG_SDL
-#ifdef __APPLE__
+#if defined(__APPLE__) || defined(main)
 #include <SDL.h>
 int qemu_main(int argc, char **argv, char **envp);
 int main(int argc, char **argv)
 {
-    qemu_main(argc, argv, NULL);
+    return qemu_main(argc, argv, NULL);
 }
 #undef main
 #define main qemu_main