Update libmpg123

Updated libmpg123 to the latest version (version 1.15.1, dated 2013-02-23). Tested in win64 and android. Note the extra android compiler switch to define the assembler byte alignment, which might be also needed for other compilers (iOS?).

This might resolve issue 1301
http://code.google.com/p/libgdx/issues/detail?id=1301