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/entrypoint.sh

@@ -4,5 +4,17 @@ set -e
 # Source Emscripten environment
 source /emsdk/emsdk_env.sh 2>/dev/null
 
+# Sync source from host to workspace volume
+# This fixes macOS Docker timestamp issues with bind mounts
+# (autoconf sanity checks fail when timestamps appear inconsistent)
+if [[ -d /workspace-host ]]; then
+    echo "Syncing source code to container volume..."
+    rsync -a --delete \
+        --exclude='build-wasm' \
+        --exclude='output' \
+        /workspace-host/ /workspace/
+    echo "Sync complete."
+fi
+
 # Execute command or start shell
 exec "$@"