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