The `struct timespec` type documented as being passed to the `futex()`
syscall actually needs to be the *kernel’s* timespec type. This will be
a different width from the userspace timespec type if running a 64-bit
userspace on a 32-bit kernel.

That mismatch will cause `g_cond_wait_until()` to return `FALSE`
immediately.

No other uses of `futex()` in GLib use the timeout argument, so they’re
all OK.

Following a detailed suggestion by Rich Felker, pass a different
timespec type into `futex()` if `__NR_futex_time64` is defined. That’s
the 64-bit time version of `futex()` which was added in kernel 5.1, and
which was only added for 32-bit kernels.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #2634