buildroot

pulseaudio: normalize alsa-lib optional dependency

Put together alsa-lib dependency and configure option code. As a side
effect we now avoid alsa-lib dependency when the required support in
alsa is missing.

Use positive logic.

Explicitly enable alsa support when available.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>