Add -fno-rtti to build instructions for lesson 15
Add -fno-rtti to build instructions for lesson 15

Fixes #981

adasworks/Halide,adasworks/Halide,jiawen/Halide,dougkwan/Halide,jiawen/Halide,ronen/Halide,jiawen/Halide,adasworks/Halide,kgnk/Halide,psuriana/Halide,tdenniston/Halide,dougkwan/Halide,adasworks/Halide,psuriana/Halide,ronen/Halide,ronen/Halide,jiawen/Halide,jiawen/Halide,dougkwan/Halide,dougkwan/Halide,kgnk/Halide,tdenniston/Halide,kgnk/Halide,adasworks/Halide,psuriana/Halide,dougkwan/Halide,kgnk/Halide,tdenniston/Halide,ronen/Halide,psuriana/Halide,psuriana/Halide,tdenniston/Halide,ronen/Halide,dougkwan/Halide,tdenniston/Halide,tdenniston/Halide,dougkwan/Halide,ronen/Halide,psuriana/Halide,adasworks/Halide,adasworks/Halide,kgnk/Halide,jiawen/Halide,kgnk/Halide,ronen/Halide,kgnk/Halide,dougkwan/Halide,ronen/Halide,jiawen/Halide,adasworks/Halide,tdenniston/Halide,tdenniston/Halide,psuriana/Halide,kgnk/Halide