Update build.gradle for build native code with NDK

Because of Android Studio/Gradle keeps evolution, and the special need of Android NDK's configuration, sometimes it is a little difficult to get things working smoothly.

There is some references like:

https://riptutorial.com/android/example/13985/enable-experimental-ndk-plugin-support-for-gradle-and-androidstudio

https://stackoverflow.com/questions/16667903/android-studio-gradle-and-ndk

https://android-developers.googleblog.com/2020/02/native-dependencies-in-android-studio-40.html

Will see some mismatch between above posts, or things not working with Android Studio, such as with Android Studio 4.1.2, Gradle model 6.5, the ndk setting in above first link will lead to weird error. The 2nd link from stackoverflow has the correct setting, i.e. ndk setting has to be put under defaultConfig, specially the abiFilters setting. As explained in the 3rd link, when deal with native code, frequently may need to link with prebuilt lib, which may only support one special ABI such as armeabi-v7a, but not all others such as 64 bit or x86. Here is an example app build.gradle file for android-eye:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 14
    buildToolsVersion "30.0.3"
    ndkVersion "17.2.4988734"

    defaultConfig {
        applicationId "teaonly.droideye"
        targetSdkVersion 14
        minSdkVersion="14"
        ndk {
            abiFilters 'armeabi-v7a'
            // 64-bit support requires an Android API level higher than 19; Namely 21 and higher
            //abiFilters 'armeabi', 'armeabi-v7a', 'arm64-v8a', 'x86', 'x86_64'
            //cppFlags.add("-O2 -Werror -Wall")
            //ldLibs.addAll(['log', 'z', 'ld'])
            stl = "c++_static"
        }

    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
        }
    }
    externalNativeBuild {
        ndkBuild {
            path file('src/main/jni/Android.mk')
        }
    }
}

dependencies {
    implementation files('libs/java_websocket.jar')
}

Building under Android Studio may hide the detail of the build, which makes it difficult to root cause when something goes wrong.

Beside gradle related setting, sometimes developer might get sysroot setting issue. Such as compiling x264 with NDK (specially old version) sometimes need some tweak, such as sysroot setting. Refer to SO1, SO2, and BuildSystemMaintainers for latest new NDK. For old NDK, at compile time sysroot is $NDK/sysroot, and at link time sysroot points to $NDK/platforms/android-$API/arch-$ARCH. One way is refer to standalone_toolchain, run $NDK/build/tools/make_standalone_toolchain.py --arch arm --api 14 --install-dir /tmp/my_toolchain to create a standalone toolchain. May need to specify CFLAGS=-D__ANDROID_API__=$API as mentioned here. The other possible way is refer to this: Using clang instead of gcc also works, e.g. for autotools-based projects CC=arm-linux-androideabi-clang CXX=arm-linux-androideabi-clang++ ./configure --cross-prefix=arm-linux-androideabi- ... works without disabling unified headers and without specifying __ANDROID_API__. 

With new version NDK,  'Application.mk' has to be provided. Also, NDK will assume the 'Application.mk' and 'Android.mk' are all located under a 'jni' sub-folder. If that is not the case, then user will have to specify the path of 'Application.mk' while invoking 'ndk-build' script, by setting NDK_APPLICATION_MK, and in 'Application.mk', user has to specify the path of 'Android.mk' by set 'APP_BUILD_SCRIPT'. Refer to https://developer.android.com/ndk/guides/ndk-build for more detail.