Merge remote-tracking branch 'remotes/kraxel/tags/audio-20190124-pull-request' into...

audio: pc speaker init fix, rework driver probing

# gpg: Signature made Thu 24 Jan 2019 13:19:52 GMT
# gpg:                using RSA key 4CB6D8EED3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>"
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>"
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>"
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/audio-20190124-pull-request:
  audio: probe audio drivers by default
  audio: error message tweak
  audio: check for pulseaudio daemon pidfile
  audio: use try-sdl for openbsd
  audio: allow optional audio drivers.
  audio: use pkg-config
  audio: fix pc speaker init

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