Yet another solution:
I got inside objects.mk file
################################################################################
# Automatically-generated file. Do not edit!
################################################################################
USER_OBJS := /home/../mylib.so
LIBS := -lstdc++fs -lGL -lGLU -lGLEW -lglut -lm -lmylib
then didn't read first line. Then altered next line. It was another projects' folder because I copied this using "copy/clone project" feature and this was causing the error for me. I changed myLib.so into /proper_address/reallyMyLib.so and it worked.
Warning: It may harm some unknown places! Backup whole project before doing this. Because it says "do not edit".