Merge remote-tracking branch 'remotes/kraxel/tags/audio-20190228-pull-request' into...

Merge remote-tracking branch 'remotes/kraxel/tags/audio-20190228-pull-request' into staging

audio: fixes and cleanups.

# gpg: Signature made Thu 28 Feb 2019 10:08:44 GMT
# gpg:                using RSA key 4CB6D8EED3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>" [full]
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>" [full]
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>" [full]
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/audio-20190228-pull-request:
  audio/sdlaudio: Simplify the sdl_callback function
  audio/sdlaudio: Remove the semaphore code
  audio: don't build alsa and sdl by default on linux
  audio: Do not check for audio_calloc failure
  audio: Use g_strdup_printf instead of manual building a string

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>