meson: define symbols for all available audio drivers

All drivers are now built by default if the corresponding libraries
are available, similar to how all other modules behave;
--audio-drv-list only governs the default choice of the audio driver.

Adjust the CONFIG_AUDIO_* preprocessor symbols so that they are
based on library availability rather than --audio-drv-list, so that
the tests and -audiodev help follow the new logic.

Cc: Gerd Hoffman <kraxel@redhat.com>
Cc: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20211007130630.632028-5-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

meson.build

@@ -1217,6 +1217,9 @@ if have_system
     'pa': pulse.found(),
     'sdl': sdl.found(),
   }
+  foreach k, v: audio_drivers_available
+    config_host_data.set('CONFIG_AUDIO_' + k.to_upper(), v)
+  endforeach
 
   # Default to native drivers first, OSS second, SDL third
   audio_drivers_priority = \
@@ -1239,9 +1242,6 @@ if have_system
     endif
   endforeach
 endif
-foreach k: audio_drivers_selected
-  config_host_data.set('CONFIG_AUDIO_' + k.to_upper(), true)
-endforeach
 config_host_data.set('CONFIG_AUDIO_DRIVERS',
                      '"' + '", "'.join(audio_drivers_selected) + '", ')