Allow libmad to be compiled in Windows.

This removes the need for an externally built library, thus officially removing the deprecated Xbox projects.

Other minimal changes were made to ensure consistency. Comments will be made in the PR.

Thanks to @Prcuvu for the initial push.

src/RageSoundReader_MP3.cpp

@@ -9,14 +9,7 @@
 #include <cerrno>
 #include <map>
 
-#if defined(_WINDOWS) || defined(MACOSX)
-#include "../extern/mad-0.15.1b/mad.h"
-#ifdef _MSC_VER
-#pragma comment(lib, "libmad.lib")
-#endif //_MSC_VER
-#else
-#include <mad.h>
-#endif // _WINDOWS
+#include "mad.h"
 
 // ID3 code from libid3:
 enum tagtype {