Building libc does not work if cygwin is installed

This is a known issue.  Please see this wiki article.

Thanks and regards,

-George

const char *mkdtemp(char pattern[])
{
    /* The pattern MUST end with six X's */

    char *start;
    if ((start = strstr(pattern, "XXXXXX")) == NULL || *(start+6) != '\0')
    {
        errno = EINVAL;
        return NULL;
    }

    /*-----------------------------------------------------------------------*/
    /* This algorithm is weak, but better than nothing.  We don't expect     */
    /* expect much danger from collisions (fingers crossed).  Hopefully      */
    /* we'll get a real version of mkdtemp eventually.                       */
    /*-----------------------------------------------------------------------*/
    srand(time(NULL));

    for (int attempt = 0; attempt 

            const char *chars =
//                "abcdefghijklmnopqrstuvwxyz"
                "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
                "0123456789";

It seems your code sample was truncated.  Could you describe explicitly what it is that you changed and why it helps?  I think you're implying that you got a filename collision, which should be very rare.