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

build.rs

@@ -1,11 +1,40 @@
 use std::env;
 use std::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.
+///
+/// This function detects the Android NDK root from well-known environment
+/// variables (the same ones CMake itself checks).
+fn detect_android_ndk() -> Option<PathBuf> {
+    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);
+            }
+        }
+    }
+    None
+}
+
 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());