audio: catch missing sdl support

sdl is probed before audio, so we can simply look at $sdl so see
whenever we have support or not.  Throw an error in case sdl audio
is requested without sdl being available.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1490000743-3615-1-git-send-email-kraxel@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/configure b/configure
index b9a30cf..1e0d7a8 100755
--- a/configure
+++ b/configure
@@ -2888,6 +2888,12 @@
     audio_pt_int="yes"
     ;;
 
+    sdl)
+    if test "$sdl" = "no"; then
+        error_exit "sdl not found or disabled, can not use sdl audio driver"
+    fi
+    ;;
+
     coreaudio)
       libs_softmmu="-framework CoreAudio $libs_softmmu"
     ;;
@@ -2901,8 +2907,8 @@
       libs_softmmu="$oss_lib $libs_softmmu"
     ;;
 
-    sdl|wav)
-    # XXX: Probes for CoreAudio, DirectSound, SDL(?)
+    wav)
+    # XXX: Probes for CoreAudio, DirectSound
     ;;
 
     *)