improve: pre-generate BPF skeleton into source

This removes requirment for setting up libelf and zlib on build
platform.

Cargo.toml

@@ -23,10 +23,16 @@ ipv6 = []
 # Required on 32-bit platforms
 bindgen = ["libbpf-sys/bindgen"]
 # Link against static `libelf` and `zlib`.
+# Notice libbpf is vendrored and static in any case.
 static = ["libbpf-sys/static"]
-#
-# libbpf is vendrored and static in any case.
-#
+# Generate BPF skeleton instead of using pre-genrated skeleton, this is for BPF
+# program development.
+gen-skel = []
+# Pre-generate BPF skeleton into source, this for release so downstream does not
+# have to generate BPF skeleton in thier build process hence introducing
+# build platform dependencies on libelf and zlib on which might be tricky to
+# handle in cross-compilation.
+gen-skel-source = ["gen-skel"]
 
 [dependencies]
 anyhow = "1.0.86"

build.rs

@@ -1,29 +1,35 @@
-use std::env;
-use std::path::PathBuf;
+fn main() {
+    #[cfg(feature = "gen-skel")]
+    {
+        use libbpf_cargo::SkeletonBuilder;
+        use std::env;
+        use std::path::PathBuf;
 
-use libbpf_cargo::SkeletonBuilder;
+        const SRC: &str = "src/bpf/einat.bpf.c";
 
-const SRC: &str = "src/bpf/einat.bpf.c";
+        let mut out = if cfg!(feature = "gen-skel-source") {
+            PathBuf::from("src")
+        } else {
+            PathBuf::from(env::var_os("OUT_DIR").expect("OUT_DIR must be set in build script"))
+        };
 
-fn main() {
-    let mut out =
-        PathBuf::from(env::var_os("OUT_DIR").expect("OUT_DIR must be set in build script"));
-    out.push("einat.skel.rs");
+        out.push("einat.skel.rs");
 
-    let mut c_args = vec![
-        "-Wno-compare-distinct-pointer-types".to_string(),
-        "-mcpu=v3".to_string(),
-    ];
+        let mut c_args = vec![
+            "-Wno-compare-distinct-pointer-types".to_string(),
+            "-mcpu=v3".to_string(),
+        ];
 
-    if cfg!(feature = "ipv6") {
-        c_args.push("-DFEAT_IPV6".to_string());
-    }
+        if cfg!(feature = "ipv6") {
+            c_args.push("-DFEAT_IPV6".to_string());
+        }
 
-    SkeletonBuilder::new()
-        .source(SRC)
-        .clang_args(c_args)
-        .debug(true)
-        .build_and_generate(&out)
-        .unwrap();
-    println!("cargo:rerun-if-changed={SRC}");
+        SkeletonBuilder::new()
+            .source(SRC)
+            .clang_args(c_args)
+            .debug(true)
+            .build_and_generate(&out)
+            .unwrap();
+        println!("cargo:rerun-if-changed={SRC}");
+    }
 }