Add an optional environment variable to require libpulse

[mutexlox-signal]

If a client is building cubeb-rs and specifically requires the pulse backend (the only tier-1 linux backend as of mid 2026), allow them to request a build failure if it's missing.

Prior to this change, missing libpulse-dev at build time would result in runtime errors initializing cubeb. This moves that error to build time and makes it more explicit.

[kinetiknz]

Thanks. Would using an env var rather than a cargo features still work for you? Something like checking for LIBCUBEB_SYS_REQUIRE_PULSE in build.rs - that matches the existing LIBCUBEB_SYS_USE_PKG_CONFIG style and avoids messing with cargo features.

[mutexlox-signal]

I think that should work, yeah. I can update that in a bit

[kinetiknz]

Thanks!