Improve and enhance WITH_MP3.

* All OSes can now take advantage of it.
* Mad is not linked if not utilized.
* Mac OS X side has a cleaner library list setup now.

src/RageSoundReader_FileReader.cpp

@@ -9,7 +9,7 @@
 #include "RageSoundReader_WAV.h"
 #endif
 
-#ifndef NO_MP3_SUPPORT
+#if defined(HAS_MP3)
 #include "RageSoundReader_MP3.h"
 #endif
 
@@ -25,7 +25,8 @@ RageSoundReader_FileReader *RageSoundReader_FileReader::TryOpenFile( RageFileBas
 	if( !format.CompareNoCase("wav") )
 		Sample = new RageSoundReader_WAV;
 #endif
-#ifndef NO_MP3_SUPPORT
+
+#if defined(HAS_MP3)
 	if( !format.CompareNoCase("mp3") )
 		Sample = new RageSoundReader_MP3;
 #endif
@@ -57,7 +58,7 @@ RageSoundReader_FileReader *RageSoundReader_FileReader::TryOpenFile( RageFileBas
 	 * wrong file format.  The error message always looks like "unknown file format" or
 	 * "Not Vorbis data"; ignore it so we always give a consistent error message, and
 	 * continue trying other file formats.
-	 * 
+	 *
 	 * OPEN_FATAL_ERROR: Either the file was opened successfully and appears to be the
 	 * correct format, but a fatal format-specific error was encountered that will probably
 	 * not be fixed by using a different reader (for example, an Ogg file that doesn't