HZ11WIP1-3:
- optimize build.sh for Tomatoware, including making fully-static build
- optimize for performance in default and static builds
- add optional HTTP 204 reply for generate_204 URLs (prevents wifi walled garden notifications on Android)
- attempt to fix pipe hangs by reverting to blocking write mode (but reads are still non-blocking)
- define _GNU_SOURCE unconditionally (better safe than sorry)