remove redundant comparison

Closes PR 14296
on BSDs, avoid `seconds->date` conversion that won't fit

On FreeBSD and related systems, calling localtime() on an out-of-range
value (e.g., where the year doesn't fit in 32 bits) still returns a
tm pointer, instead or returning NULL for an error. To avoid this
problem, restrict the argument to 51 bits (which seems like a lot
of bits, but is still confortably away from the overflow point).

Closes PR 14305
configure test for powl(), which is used for extflonums