Compile the program with:
g++ -Wall -Wextra -Werror -c main.cpp -o main.o
^^^^^^^^^^^^^^^^^^^^ <- For listing all warnings when your code is compiled.
as cout is present in the C++ standard library, which would need explicit linking with -lstdc++ when using gcc; g++ links the standard library by default.
With gcc, (g++ should be preferred over gcc)
gcc main.cpp -lstdc++ -o main.o