The src/ directory is mounted from the host in Docker-based CI, so it
is owned by the runner user and not writable by the container user.
Moving the output to build/ avoids permission conflicts because Docker
creates build/ fresh and owns it.

- Add -I$(BUILD_DIR) to CFLAGS so headers are found by the compiler.
- Add --output-dir option to make_method_table.rb.
- Accept an optional output path argument in generate_unicode_case_tables.rb.
- Simplify the autogen Make target: drop rm -f and mkdir -p $(BUILD_DIR).
- clean_all now just delegates to clean (autogen files are in build/).
- Update autogen.yml to copy from build/ when packaging release archives.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>