prepare (1/3): 
finished

make (2/3): 
g++ -g -Wall -std=c++11 main.cpp -o main -O2
/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
Makefile:2: recipe for target 'main' failed
make: *** [main] Error 1

build failed