Hello, professor AKAMA,

My name is Rodrigo Basilio and I am the main developer of Friend Engine.
Thanks for using Friend Engine.
We opted to recreate these libraries bacause in some Linux installations the user did not have these libraries for some reason. Dont bother about these warnings.

Please add this include line in the main.cpp file

#include < cstring >
or 
#include < string.h >

to get rid of the error. I will update the github to correct this error for new users.

Best,
Rodrigo

PS.: Feel free to send more questions if you need.