Fix FreeBSD build by making libbpf-cargo optional

- Make libbpf-cargo an optional build dependency
- Include it in ebpf feature to only build when needed
- Add test workflow for FreeBSD builds that can be manually triggered
- This prevents libbpf-sys from being built when cross-compiling to FreeBSD

.github/workflows/test-freebsd.yml

@@ -0,0 +1,60 @@
+name: Test FreeBSD Build
+
+on:
+  workflow_dispatch:
+  pull_request:
+    paths:
+      - 'Cargo.toml'
+      - 'build.rs'
+      - 'src/**'
+      - '.github/workflows/test-freebsd.yml'
+
+env:
+  RUST_BACKTRACE: 1
+
+jobs:
+  test-freebsd-build:
+    name: Test FreeBSD x64 Build
+    runs-on: ubuntu-22.04
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v6
+
+      - name: Install Linux dependencies
+        run: |
+          sudo apt-get update -y
+          sudo apt-get install -y libpcap-dev pkg-config
+
+      - name: Install Rust
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: stable
+          targets: x86_64-unknown-freebsd
+
+      - name: Install cross
+        run: cargo install cross@0.2.5
+
+      - name: Build for FreeBSD (without eBPF)
+        env:
+          RUSTFLAGS: "-C strip=symbols"
+        run: |
+          echo "Building for FreeBSD target without default features (no eBPF)"
+          cross build --verbose --release --target x86_64-unknown-freebsd --no-default-features
+
+      - name: Verify binary was created
+        run: |
+          if [ -f "target/x86_64-unknown-freebsd/release/rustnet" ]; then
+            echo "✅ FreeBSD binary successfully built"
+            ls -lh target/x86_64-unknown-freebsd/release/rustnet
+          else
+            echo "❌ FreeBSD binary not found"
+            exit 1
+          fi
+
+      - name: Upload binary as artifact
+        uses: actions/upload-artifact@v5
+        with:
+          name: rustnet-freebsd-x64
+          path: target/x86_64-unknown-freebsd/release/rustnet
+          if-no-files-found: error

Cargo.toml

@@ -85,7 +85,7 @@ windows = { version = "0.62", features = [
 ] }
 
 [target.'cfg(target_os = "linux")'.build-dependencies]
-libbpf-cargo = "0.25"
+libbpf-cargo = { version = "0.25", optional = true }
 
 [features]
 # eBPF is enabled by default for enhanced performance on Linux.
@@ -93,7 +93,7 @@ libbpf-cargo = "0.25"
 # and dependencies are Linux-specific (guarded by target_os checks).
 default = ["ebpf"]
 linux-default = ["ebpf"]  # Deprecated: kept for backwards compatibility
-ebpf = ["libbpf-rs", "bytes", "libc"]
+ebpf = ["libbpf-rs", "bytes", "libc", "dep:libbpf-cargo"]
 
 # Minimal cross configuration to override dependency conflicts
 [workspace.metadata.cross.build.env]