xtime_cmp: use comparison not subtraction

[phhodges]

Subtraction of integral types is prone to arithmetic
overflow.
It is not well-behaved for implementing comparison unless
we first cast both operands to a wider integral type or
assert that they fall in a narrower range.

Choose comparison order to slightly favour future timeouts,
or imminent timeouts in the same second?

Patch provided "as is" without express or implied warranty.
It may contain any number of intended and unintended flaws.

[martin_wille]

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This was fixed in version 1.14 (2004-08-09) of xtime.hpp
(Boost 1.32.0 and better).