build(afxdp): add USE_AF_XDP=1 Makefile conditional — Phase 2 PR C (fork)

[skullcrushercmd]

Summary

Add the USE_AF_XDP=1 Makefile block that pulls the in-tree src/send-afxdp.c (PR #1, 4dd3a2a) and src/recv-afxdp.c (PR #2, c69872f) into the build and links libxdp + libbpf + libelf. With USE_AF_XDP unset (the default), the build is unchanged — the AF_XDP TUs compile to nothing under their #ifdef USE_AF_XDP guards, no XDP libs are linked, and --io-engine=af_xdp at runtime keeps emitting the existing "rebuild with USE_AF_XDP=1" message from engine.c / conf.c.

This is Phase 2 PR C of the AF_XDP integration plan (AnyVM-Tech/AnyScan §3.6 of plans/2026-04-27-portscan-afxdp-plan-v1.md). The companion AnyScan PR C adds the apt-get deps, systemd CAP_BPF, and the runtime probe so worker bundles can detect when the AF_XDP code path is available; that one will follow shortly.

What changed

Makefile — one new conditional block, mirroring the shape of the existing USE_PFRING_ZC block:

• Adds -DUSE_AF_XDP to CFLAGS so the io_engine_af_xdp vtable in src/engine.c:146 is registered and the AF_XDP TUs compile in.
• Pulls pkg-config --cflags/--libs libxdp libbpf for include + link flags. Debian/Ubuntu ship libxdp.pc / libbpf.pc in the -dev packages, so pkg-config is the supported source. Falls back to plain -lxdp -lbpf when pkg-config returns empty (hosts where libxdp/libbpf were source-built without .pc files).
• Appends -lelf -lz unconditionally — libxdp transitively walks ELF sections and decompresses BTF, and pkg-config sometimes does not pull these through depending on how libxdp.pc was generated.
• Adds src/send-afxdp.c src/recv-afxdp.c to SRCS.

Test status

Verified on Debian (libxdp 1.5.4, libbpf 1.5.0, libelf 0.192):

make USE_AF_XDP=1:

$ make clean && make USE_AF_XDP=1
[...]
gcc src/main.o ... src/send-afxdp.o src/recv-afxdp.o -o scanner -lpthread -lm -lxdp -lbpf -lelf -lz
$ ldd scanner | grep -E 'libxdp|libbpf|libelf'
        libxdp.so.1 => /lib/x86_64-linux-gnu/libxdp.so.1
        libbpf.so.1 => /lib/x86_64-linux-gnu/libbpf.so.1
        libelf.so.1 => /lib/x86_64-linux-gnu/libelf.so.1
$ nm scanner | grep -E 'io_engine_af_xdp|xdp_(sender|receiver)_thread'
000000000000dd40 D io_engine_af_xdp
0000000000009ad0 T xdp_receiver_thread
00000000000090c0 T xdp_sender_thread
$ make USE_AF_XDP=1 test
=== Results: 11 passed, 0 failed ===

Default build (USE_AF_XDP unset):

$ make clean && make
[...no AF_XDP libs in link line...]
$ ldd scanner | grep -E 'libxdp|libbpf|libelf' || echo none
none
$ make test
=== Results: 11 passed, 0 failed ===

Smoke tests cover both build flag states — see tests/io_engine_dispatch.sh binary_has_af_xdp branch — so this PR exercises both modes of the dispatch path that PR #1 + PR #2 added.

Out of scope (deferred)

• AnyScan apt-get deps (libxdp-dev libbpf-dev libelf-dev), worker systemd unit CAP_BPF, and the install-time runtime probe — these land in the companion AnyScan PR C.
• Runtime opt-in (ANYSCAN_SCANNER_IO_ENGINE env, adapter --io-engine plumbing) — Phase 2 PR D, after PR C lands.
• Live c6in.metal bench — Phase 2 PR E, owned out of session.

Plan reference

plans/2026-04-27-portscan-afxdp-plan-v1.md §3.6 ("Build-system integration").