Oh wow, it works!!!
I installed g++-10 and gcc-10 with:
sudo apt install g++-10
sudo apt install gcc-10
Then I ran this command:
sudo make CC=gcc-10 CPP=g++-10 CXX=g++-10 LD=g++-10
with error, because I had to run ./configure command again. So I did and then tried it again.
After that I've finally built retroarch core package and performed basic installation of Retropie.
I could not figure it out myself. Thank you for your direction, you're the best! ;)