Fix opt_imax() call

Not all architectures have a time_t defined the same way.  To make
sure we get the same result, we need to cast &checkoffset to (intmax_t *)
and make sure that intmax_t is defined somehow.

To make really sure we don't pass a variable with the wrong size down
to opt_imax(), we use a temporary intmax_t.

Reviewed-by: Rich Salz <rsalz@openssl.org>