Add core and plugin module of shadowsocks-android to my app

develop
question

#1

I want to use SOCKS5 for VPN connection in my app.

I did git clone --recurse-submodules https://github.com/shadowsocks/shadowsocks-android
After cloning repo I have added core and plugin module by importing to my app.

I am not able to build the app and getting this error

Build command failed.
Error while executing process /path/to/Android/sdk/ndk-bundle/ndk-build with arguments {NDK_PROJECT_PATH=null APP_BUILD_SCRIPT=/path/to/app/root/socks5-core/src/main/jni/Android.mk NDK_APPLICATION_MK=/path/to/app/root/socks5-core/src/main/jni/Application.mk APP_ABI=x86 NDK_ALL_ABIS=x86 NDK_DEBUG=1 APP_PLATFORM=android-16 NDK_OUT=/path/to/app/root/sentinel_android/socks5-core/build/intermediates/ndkBuild/debug/obj NDK_LIBS_OUT=/path/to/app/root/socks5-core/build/intermediates/ndkBuild/debug/lib -j4 /path/to/app/root/socks5-core/build/intermediates/ndkBuild/debug/obj/local/x86/libtun2socks.so}
[x86] Executable     : libtun2socks.so
/path/to/app/root/socks5-core/src/main/jni/badvpn/system/BUnixSignal.c:172: error: undefined reference to 'signalfd'
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [/path/to/app/root/socks5-core/build/intermediates/ndkBuild/debug/obj/local/x86/libtun2socks.so] Error 1

Everything is working fine in the repository as I cloned with recursive submodules.

Can someone please help me to fix the issue?
Can someone please help me add only core and plugin module to my app via git submodule?