Fix mktime64 failure on Travis and Linux

[schwern]

t/mktime64.t ............... # Failed test
# mktime64(140721847750144)
# have: 12344592500
# want: 12344678900

It's test 13 in t/mktime64.t.c which is this.

is_Int64( mktime64(&date), time, "mktime64(%lld)", time );

Time is set earlier as time = 12344678900LL;. So a value of 140721847750144 is absurd. It should be mktime64(12344678900). It's possible something is altering time.