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>