snapcast: define ALSA support explicitly

snapcast: define ALSA support explicitly

As Hannu Nyman pointed out, in some cases the build of the package is
able to fail:

Package snapserver is missing dependencies for the following libraries:
libasound.so.2

He said: on some build runs, alsa is built (as required by other
packages), and if it is built before snapcast, it may be detected,
if the feature is not explicitly disabled in CMakeLists.txt.

Also: "I think that you could drop that AUDIO_SUPPORT conditional.
AUDIO_SUPPORT is selected e.g. if USB_SUPPORT is selected,
so it is not that big restriction."

Suggested-by: Hannu Nyman <[email protected]>
Signed-off-by: Szabolcs Hubai <[email protected]>