Make using mpg123 optional, add option to linux build system

Added so people can make patent-free builds (easily), for example for patent-free distro's

src/modules/sound/lullaby/Mpg123Decoder.cpp

@@ -18,6 +18,8 @@
 * 3. This notice may not be removed or altered from any source distribution.
 **/
 
+#ifndef LOVE_NOMPG123
+
 #include "Mpg123Decoder.h"
 
 #include <common/Exception.h>
@@ -229,3 +231,5 @@ namespace lullaby
 } // lullaby
 } // sound
 } // love
+
+#endif // LOVE_NOMPG123

src/modules/sound/lullaby/Mpg123Decoder.h

@@ -18,6 +18,7 @@
 * 3. This notice may not be removed or altered from any source distribution.
 **/
 
+#ifndef LOVE_NOMPG123
 #ifndef LOVE_SOUND_LULLABY_LIBMPG123_DECODER_H
 #define LOVE_SOUND_LULLABY_LIBMPG123_DECODER_H
 
@@ -77,3 +78,4 @@ namespace lullaby
 } // love
 
 #endif // LOVE_SOUND_LULLABY_LIBMPG123_DECODER_H
+#endif // LOVE_NOMPG123

src/modules/sound/lullaby/Sound.cpp

@@ -37,7 +37,9 @@ namespace lullaby
 
 	Sound::~Sound()
 	{
+#ifndef LOVE_NOMPG123
 		Mpg123Decoder::quit();
+#endif // LOVE_NOMPG123
 	}
 
 	const char * Sound::getName() const
@@ -55,8 +57,10 @@ namespace lullaby
 		// Find a suitable decoder here, and return it.
 		if (ModPlugDecoder::accepts(ext))
 			decoder = new ModPlugDecoder(data, ext, bufferSize);
+#ifndef LOVE_NOMPG123
 		else if (Mpg123Decoder::accepts(ext))
 			decoder = new Mpg123Decoder(data, ext, bufferSize);
+#endif // LOVE_NOMPG123
 		else if (VorbisDecoder::accepts(ext))
 			decoder = new VorbisDecoder(data, ext, bufferSize);
 		/*else if (FLACDecoder::accepts(ext))