The validation end of XDM-AUTHORIZATION-1 needs to check that two
time_t values differ by at most XDM_MAXSKEW, which it was doing by
subtracting them and passing the result to abs(). This provoked a
warning from OS X's clang, on the reasonable enough basis that the
value passed to abs was unsigned.
Fixed by using the (well defined) unsigned arithmetic wraparound: to
check that the mathematical difference of two unsigned numbers is in
the interval [-k,+k], compute their difference _plus k_ as an
unsigned, and check the result is in the interval [0,2k] by doing an
unsigned comparison against 2k.
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