snapcast: make build options explicit and per-package deps precise

snapcast: make build options explicit and per-package deps precise

Pin every CMake BUILD_WITH_* switch and expose the optional features as
config symbols, so builds are reproducible and snapserver/snapclient
each depend only on the libraries their binary actually links (boost is
header-only and becomes a build-only dependency).

Add a patch introducing a BUILD_WITH_SOXR option: upstream looks SOXR up
with an unconditional pkg_search_module(), so resampling gets enabled
purely by whether libsoxr happens to be discoverable in the shared
staging dir - non-deterministic in a package feed. The option makes it
an explicit, REQUIRED choice (enabled-but-missing fails at configure
time instead of silently dropping the feature).

Assisted-By: Claude Opus 4.8 (1M context) <redacted>
Signed-off-by: Mirko Vogt <redacted>