From the Input/mpg123/README file in xmms-0.9.1:

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Note about the mpg123 licens!

We have permission to release mpg123 source inside xmms as GPL! Using
mpg123 source (at least in a commercial environment) may infringe 3rd
party patents. Also Michael Hipp (hippm@informatik.uni-tuebingen.de) would
like to know if you modify this sourcecode. send him an email if you do,
he would like to have an eye on external developments.

Happy playing!
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Thus, #include <gpl.h>.
