bug#11825: 24.1.50; float-time no longer accepts negative time values

[Paul Eggert]

On my host (Fedora 15 x86-64) float-time works just fine
with negative values.  Possibly the problem is that
time_t is unsigned on your host, and that float-time converts
its argument through time_t, which fails.  As far as I
can see Emacs has behaved this way for some time,
but it doesn't hurt to generalize float-time so that
it does not fail due to out-of-range integers, so I
installed that into the trunk as bzr 108928.  Please
give it a try on your host to see whether it fixes
the problem.