Tested using `avr-gcc` version 8.3.0 with: `./configure --host=avr --enable-16bit --disable-filesystem --enable-singlethreaded CFLAGS="-DWOLFSSL_GENSEED_FORTEST -DWOLFSSL_USER_CURRTIME -DWOLFSSL_USER_IO -DWOLFSSL_NO_SOCK -DNO_WRITEV -DUSER_TICKS" --enable-tls13 --enable-compkey --enable-certgen --enable-certext --enable-keygen --enable-session-ticket --enable-maxfragment`. Also with `--enable-fastmath`.
The wolfSSL manual is available at: http://www.wolfssl.com/documentation/wolfSSL-Manual.pdf