add libsoundio audio backend

This adds libsoundio (http://libsound.io/) as an available audio
backend. libsoundio supports JACK, PulseAudio, ALSA, CoreAudio,
WASAPI, and a dummy backend.

cmake/modules/FindSoundIo.cmake

@@ -0,0 +1,16 @@
+# Copyright (c) 2015 Andrew Kelley
+# This file is MIT licensed.
+# See http://opensource.org/licenses/MIT
+
+# SOUNDIO_FOUND
+# SOUNDIO_INCLUDE_DIR
+# SOUNDIO_LIBRARY
+
+find_path(SOUNDIO_INCLUDE_DIR NAMES soundio/soundio.h)
+
+find_library(SOUNDIO_LIBRARY NAMES soundio)
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(SOUNDIO DEFAULT_MSG SOUNDIO_LIBRARY SOUNDIO_INCLUDE_DIR)
+
+mark_as_advanced(SOUNDIO_INCLUDE_DIR SOUNDIO_LIBRARY)