feat: support cross-compilation (tested for 32-bit ARM Linux)

[ibrahim-mubarak]

What

Makes the C++ SDK buildable with a CMake cross toolchain file. Verified end-to-end for 32-bit ARM Linux (armv7-unknown-linux-gnueabihf) targeting an embedded armv8 AArch32 (Cortex-A32, glibc 2.39) sysroot: liblivekit.so + liblivekit_ffi.so build, and livekit::initialize()/shutdown() round-trips through the FFI under qemu-arm.

Companion PR (Rust side / libwebrtc arm build): livekit/rust-sdks#1218

Changes

• Honor a pre-set RUST_TARGET_TRIPLE: a cross toolchain file can now set it (cache var), and the cargo build gets --target plus the imported liblivekit_ffi.so path follows target/<triple>/. Previously the triple was only derived on macOS.
• Scope cargo to -p livekit-ffi: the workspace has no default-members, so plain cargo build also compiled all examples (including wgpu-based ones) — wasted work natively, build failures when cross-compiling.
• Cross-aware protoc: cmake/protobuf.cmake force-enabled protobuf_BUILD_PROTOC_BINARIES, producing a target-arch protoc that cannot run on the build host ("Exec format error" at codegen). When CMAKE_CROSSCOMPILING, protoc binaries are now skipped and a host protoc matching the vendored protobuf version must be passed via -DProtobuf_PROTOC_EXECUTABLE (official release binaries work). It is forwarded to the Rust build via ENV{PROTOC} as before.

Usage example

# toolchain-arm.cmake
set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR arm)
set(CMAKE_SYSROOT /path/to/target-sysroot)
set(CMAKE_C_COMPILER   arm-...-gcc)
set(CMAKE_CXX_COMPILER arm-...-g++)
set(RUST_TARGET_TRIPLE armv7-unknown-linux-gnueabihf CACHE STRING "" FORCE)

cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Release \
  -DCMAKE_TOOLCHAIN_FILE=toolchain-arm.cmake \
  -DProtobuf_PROTOC_EXECUTABLE=/path/to/host/protoc   # v25.3
cmake --build build

(plus the usual cargo cross setup: rustup target add, linker in .cargo/config.toml, and LK_CUSTOM_WEBRTC pointing at a libwebrtc built with build_linux.sh --arch arm from the rust-sdks PR.)

No behavior change for native builds.

🤖 Generated with Claude Code

https://claude.ai/code/session_01L4fDRyTiTphQnidL2Myptq

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[ibrahim-mubarak]

Closing for now — will resubmit once ready.