fix: set CMAKE_ANDROID_NDK for Android cross-compilation

cmake-rs sets CMAKE_SYSTEM_NAME=Android but not CMAKE_ANDROID_NDK,
causing CMake >= 3.21 to fail with 'Neither the NDK or a standalone
toolchain was found'. Detect NDK root from environment variables and
pass it to cmake::Config.

Author: xz-dev

Cargo.toml

@@ -9,4 +9,5 @@ license = "MIT"
 
 [build-dependencies]
 cmake = "0.1"
+cc = "1"
 bindgen = "0.72"

build.rs

@@ -1,11 +1,61 @@
 use std::env;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
+
+/// When cross-compiling for Android, the `cmake` crate (cmake-rs) sets
+/// `CMAKE_SYSTEM_NAME=Android` but does **not** set `CMAKE_ANDROID_NDK`.
+/// CMake ≥ 3.21's `Platform/Android-Determine.cmake` then fails because it
+/// cannot locate the NDK.
+///
+/// Detection strategy:
+///   1. Check well-known environment variables (`ANDROID_NDK_ROOT`, etc.).
+///   2. Infer from the C compiler path that `cc` selects for this target.
+///      NDK compilers live at `<NDK>/toolchains/llvm/prebuilt/<host>/bin/…`,
+///      so we walk up from the compiler looking for the `toolchains` dir.
+fn detect_android_ndk() -> Option<PathBuf> {
+    // 1. Prefer explicit env vars (same ones CMake itself checks)
+    for var in ["ANDROID_NDK_ROOT", "ANDROID_NDK_HOME", "ANDROID_NDK"] {
+        if let Ok(val) = env::var(var) {
+            let p = PathBuf::from(&val);
+            if p.is_dir() {
+                return Some(p);
+            }
+        }
+    }
+
+    // 2. Infer from the C compiler path
+    let compiler = cc::Build::new()
+        .cargo_metadata(false)
+        .opt_level(0)
+        .warnings(false)
+        .try_get_compiler()
+        .ok()?;
+    let cc_path = compiler.path().canonicalize().ok()?;
+    let mut dir: &Path = cc_path.parent()?;
+    loop {
+        if dir.file_name().and_then(|n| n.to_str()) == Some("toolchains")
+            && dir.join("llvm").is_dir()
+        {
+            return dir.parent().map(|p| p.to_path_buf());
+        }
+        dir = dir.parent()?;
+    }
+}
 
 fn main() {
+    let target_os = env::var("CARGO_CFG_TARGET_OS").unwrap_or_default();
+
     // Build libversion static library using cmake
-    let dst = cmake::Config::new("libversion")
-        .build_target("libversion_static")
-        .build();
+    let mut cmake_cfg = cmake::Config::new("libversion");
+    cmake_cfg.build_target("libversion_static");
+
+    // Work around cmake-rs not setting CMAKE_ANDROID_NDK for Android targets.
+    if target_os == "android" {
+        if let Some(ndk_root) = detect_android_ndk() {
+            cmake_cfg.define("CMAKE_ANDROID_NDK", &ndk_root);
+        }
+    }
+
+    let dst = cmake_cfg.build();
 
     let build_dir = dst.join("build").join("libversion");
     println!("cargo:rustc-link-search=native={}", build_dir.display());