Remove warning in autotools and make LOVE_OLD_MODPLUG define that disables some new modplug settings

src/modules/sound/lullaby/ModPlugDecoder.cpp

@@ -42,8 +42,12 @@ namespace lullaby
 		// fill with modplug defaults (modplug _memsets_, so we could get
 		// garbage settings when the struct is only partially initialized)
 		// This does not exist yet on Windows.
+
+		// Some settings not supported by some older versions
+#ifndef LOVE_OLD_MODPLUG
 		settings.mStereoSeparation = 128;
 		settings.mMaxMixChannels = 32;
+#endif
 		settings.mReverbDepth = 0;
 		settings.mReverbDelay = 0;
 		settings.mBassAmount = 0;