at-compat.at: Add patch to avoid DJGPP date issue.

DJGPP's "date +%s" does not actually work like GNU, although it probably
was supposed to.  It can return a string which begins with zero.  The
testsuite only checks that the output is all digits, so accepts this as
being a working GNU-like date output.

This is then used in an arithmetic expansion, and the shell interprets
this string as octal digits, crapping out if any of those digits are 8
or 9.  If this happens (perhaps depending on the phase of the moon)
then the testsuite crashes before printing the final summary.