Sets a debug output callback called by the library when built with
``DEBUGBUILD`` macro defined.  If this option is not used, debug
log is written into standard error output.

For builds without ``DEBUGBUILD`` macro defined, this function is
noop.

Note that building with ``DEBUGBUILD`` may cause significant
performance penalty to libnghttp2 because of extra processing.  It
should be used for debugging purpose only.

.. Warning::

  Building with ``DEBUGBUILD`` may cause significant performance
  penalty to libnghttp2 because of extra processing.  It should be
  used for debugging purpose only.  We write this two times because
  this is important.