main()
{
    printf("%dn", MARCO);
//  printf("%sn", MARCO);
}

gcc -D MARCO=12345 test.c

gcc -D MARCO=abcde test.c

error: ‘abcde’ undeclared (first use in this function)

gcc -DMARCO='"abcde"' test.c

#include <stdio.h>
#define STRINGIFY(x) #x
#define MACRO(x)     STRINGIFY(x)
int main(void)
{
    printf("%sn", MACRO(MARCO));
    return(0);
}