fix SDL sound on windows

In SoundSDL, when building on windows add this init code:

```
SDL_setenv("SDL_AUDIODRIVER", "directsound", true);
```

Otherwise there is no sound on windows.

Also update SDL to 2.0.8 from 2.0.7 in build scripts.

src/common/SoundSDL.cpp

@@ -115,6 +115,11 @@ void SoundSDL::write(uint16_t * finalWave, int length) {
 bool SoundSDL::init(long sampleRate) {
     if (initialized) deinit();
 
+    // no sound on windows unless we do this
+#ifdef _WIN32
+    SDL_setenv("SDL_AUDIODRIVER", "directsound", true);
+#endif
+
     SDL_AudioSpec audio;
     SDL_memset(&audio, 0, sizeof(audio));
 

tools/builder/core.sh

@@ -165,7 +165,7 @@ DISTS=$DISTS'
     libgpg-error    https://www.gnupg.org/ftp/gcrypt/libgpg-error/libgpg-error-1.27.tar.bz2                     lib/libgpg-error.a
     libgcrypt       https://www.gnupg.org/ftp/gcrypt/libgcrypt/libgcrypt-1.8.2.tar.bz2                          lib/libgcrypt.a
     libsecret       http://ftp.gnome.org/pub/gnome/sources/libsecret/0.18/libsecret-0.18.5.tar.xz               lib/libsecret-1.a
-    sdl2            https://www.libsdl.org/release/SDL2-2.0.7.tar.gz                                            lib/libSDL2.a
+    sdl2            https://www.libsdl.org/release/SDL2-2.0.8.tar.gz                                            lib/libSDL2.a
     flac            https://github.com/xiph/flac/archive/f7cd466c24fb5d1966943f3ea36a1f4a37858597.tar.gz        lib/libFLAC.a
     libogg          http://downloads.xiph.org/releases/ogg/libogg-1.3.2.tar.xz                                  lib/libogg.a
     libvorbis       https://github.com/xiph/vorbis/archive/v1.3.5.tar.gz                                        lib/libvorbis.a