And probably -Werror (all warnings become errors, breaking compilation).

Fun post. Now I know why everyone was raving about the clang error-messages:

    echo "void main() {}" > main.c


    gcc -Wall -Werror --std=c99 main.c 
    main.c:1:6: error: return type of ‘main’ is not ‘int’ [-Werror=main]
    cc1: all warnings being treated as errors


    clang -Wall -Werror --std=c99 main.c 
    main.c:1:1: error: 'main' must return 'int'
    void main() {}
    ^
    1 error generated.

    (not showing clangs pretty colours)