Hello world C99, C11, C17, C2x

// Hello world program
/* Language:
    - C99 (ISO/IEC 9899:1999/Cor 3:2007)
    - C11 (ISO/IEC 9899:2011/Cor 1:2012)
    - C18 (ISO/IEC 9899:2018)
    - [upcoming] C2x
*/

#include <stdio.h>

int main(void)
{
    puts("Hello world!");
}