Define ANDROID_NDK CMake variable when building for Android

Versions of SDL since commit 5b65e0af01873e26673c11da43001e7075986290
fail to build if this isn't set. Let's take the value from the
ANDROID_NDK_HOME environment variable, which seems to be the standard.

sdl2-sys/build.rs

@@ -132,6 +132,10 @@ fn compile_sdl2(sdl2_build_path: &Path, target_os: &str) -> PathBuf {
         cfg.define("VIDEO_OPENGLES", "OFF");
     }
 
+    if target_os == "android" {
+        cfg.define("ANDROID_NDK", env::var("ANDROID_NDK_HOME").expect("ANDROID_NDK_HOME environment variable must be set when compiling for Android"));
+    }
+
     if cfg!(feature = "static-link") {
         cfg.define("SDL_SHARED", "OFF");
         cfg.define("SDL_STATIC", "ON");