RoarAudio's sndio emulation has been a source for annoyances.  First,
their headers turned out to be broken with C++, due to their use of
the "new" keyword.  Then they used a preprocessor macro to rename
"sio_hdl" to something else, effectively disallowing the use of
forward declarations.  Enough is enough, and I'm removing support for
it.

RoarAudio users should better use the RoarAudio output plugin.