You need the Android Native Development Kit (NDK) installed. See here to download it.
This project is tested against Android NDK version r20.
If you installed version r20 all you have to do is to expose the NDK root directory as environment variable, e.g:
export ANDROID_NDK_ROOT=$HOME/android-ndk-r20
If you installed another version you have to expose the NDK root directory as well as the NDK version, e.g:
export ANDROID_NDK_ROOT=$HOME/android-ndk-r17c
export NDK_VERSION=android-ndk-r17c
To specify the minimum sdk version set the environment variable below:
export MIN_SDK_VERSION=21 # Default value if unset
To specify the prefix directory set the environment variable below:
export ANDROID_BUILD_DIR=./builds/android/prefix/<android_arch> # Default value if unset
In the android directory, run:
./build.sh [ arm | arm64 | x86 | x86_64 ]