Assuming ndk-build
is in your PATH
ndk-build
Then you can build with gradlew
./gradlew build
You can also directly install to a plugged Android device
./gradlew installDebug
You can also open the project in Android Studio and build the app from there.
All the prebuilt libraries have been built with android NDK r21d, so they might not work properly if the main project is built using a different NDK version