Fix Docker macOS timestamp issues and improve build reproducibility

- Add rsync-based source sync for Docker builds to fix macOS VirtioFS
  timestamp inconsistencies that caused autoconf sanity checks to fail
- Add config.sub wrapper (SHELL/CONFIG_SHELL) to support emscripten/wasm32
  targets without modifying submodule files
- Fix protobuf build to use native gcc/g++ for protoc instead of Emscripten
- Add python PATH fix for macOS (Homebrew's python3 symlink) in wxWidgets build

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker/build.sh

@@ -20,6 +20,12 @@ fi
 # Start container if not running
 docker compose -f docker/docker-compose.yml up -d
 
+# Sync source code to container volume (fixes macOS Docker timestamp issues)
+# This ensures timestamps are consistent within the container filesystem
+echo "Syncing source code to container..."
+docker compose -f docker/docker-compose.yml exec kicad-wasm-builder \
+    rsync -a --delete --exclude='build-wasm' --exclude='output' /workspace-host/ /workspace/
+
 # Run build command (without asyncify - handled on host due to memory requirements)
 docker compose -f docker/docker-compose.yml exec kicad-wasm-builder \
     /workspace/scripts/kicad/build-pcbnew.sh "${ARGS[@]}"