This avoids static builds putting the "no-op" implementation of
PlaybackStream::create() in the static archive before the strong
implementation later. On ELF (and probably PE/COFF too), a weak
definition in a static archive is chosen and locked-in as the
definition before a strong definition later in the archive.
Before this change, static builds would have no audio support
at all.
The goal here is to ensure we check for audio backends in a way that
makes sense. On macOS, let's just always use Audio Unit (and thus avoid
any checks for Pulse, to reduce needless/confusing build log noise). We
will also only use the Qt audio backend if no other backend was found,
rather than only checking for Pulse.
