Backward compatibility for mixer version check (#46)

SDL_MIXER_VERSION_ATLEAST isn't available in SDL Mixer 2.0.1, which will cause build errors.

SpaceCadetPinball/Sound.cpp

@@ -1,6 +1,9 @@
 #include "pch.h"
 #include "Sound.h"
 
+#ifndef SDL_MIXER_VERSION_ATLEAST
+#define SDL_MIXER_VERSION_ATLEAST(X, Y, Z)   (SDL_MIXER_COMPILEDVERSION >= SDL_VERSIONNUM(X, Y, Z))
+#endif
 
 int Sound::num_channels;
 bool Sound::enabled_flag = false;