custom_rules.xml

<?xml version="1.0" encoding="UTF-8"?>
<project name="custom_rules">
    
    <!-- quick check on ndk.dir -->
    <target name="-check-ndk">
        <fail
            message="ndk.dir is missing. Make sure to put it in local.properties"
            unless="ndk.dir"
        />
    </target>
    
    <target name="-check-dependencies">
        <fail
            message="yasm is missing. Make ensure yasm is installed and present in /usr/bin"
            unless="yasm.exists"
        />
    </target>
    
    <target name="native-build" depends="native-clean">
        <exec executable="${ndk.dir}/ndk-build" failonerror="true" />
    </target>
    
    <target name="native-clean">
        <exec executable="${ndk.dir}/ndk-build" failonerror="true">
            <arg value="clean" />
        </exec>
    </target>
    
    <target name="build-ffmpeg">
        <exec executable="/bin/sh" dir="." failonerror="true" osfamily="unix">
            <arg value="scripts/build-ffmpeg.sh" />
        </exec>
    </target>
    
    <target name="-pre-build">
        <available file="/usr/bin/yasm" type="file" property="yasm.exists"/>
        <antcall target="-check-dependencies" />
        <echo>Building native code...</echo>
        <antcall target="-check-ndk" />
        <antcall target="build-ffmpeg" />
        <antcall target="native-build" />
        <echo>Successfully built native code</echo>
    </target>
    
</project>