Update build script to enable cross compiling for Android

Based on the cmake manual, cross compiling for Android,
https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html#cross-compiling-for-android-with-the-ndk

CMAKE_SYSTEM_NAME **must** be set to Android.

Additionally, set CMAKE_ANDROID_NDK if corresponding env var is available.
Android ecosystem uses both ANDROID_NDK_ROOT and ANDROID_NDK_HOME to refer to
NDK's path. For example, github runners has both variables set:
actions/runner-images#2426

These two cmake variables are sufficient to build for Android with cargo-ndk tool.

oqs-sys/Cargo.toml

@@ -22,6 +22,7 @@ pkg-config = "0.3"
 cmake = "0.1"
 bindgen = "0.69"
 build-deps = "0.1"
+build-target = "0.4.0"
 
 [features]
 default = ["openssl", "kems", "sigs"]

oqs-sys/build.rs

@@ -106,6 +106,18 @@ fn build_from_source() -> PathBuf {
         config.define(permit_unsupported, str);
     }
 
+    if build_target::target_os().unwrap() == build_target::Os::Android {
+        config.define("CMAKE_SYSTEM_NAME", "Android");
+
+        if let Ok(ndk) = std::env::var("ANDROID_NDK_HOME") {
+            config.define("CMAKE_ANDROID_NDK", ndk);
+            println!("cargo:rerun-if-env-changed=ANDROID_NDK_HOME");
+        } else if let Ok(ndk) = std::env::var("ANDROID_NDK_ROOT") {
+            config.define("CMAKE_ANDROID_NDK", ndk);
+            println!("cargo:rerun-if-env-changed=ANDROID_NDK_ROOT");
+        }
+    }
+
     let outdir = config.build_target("oqs").build();
 
     // lib is put into $outdir/build/lib