configure.ac: consider libusb_os_handle absence non-fatal on Windows

configure.ac: consider libusb_os_handle absence non-fatal on Windows

Keep checking for the libusb_os_handle data type's presence, but don't
consider its absence on Windows fatal. Recent mainline libusb provides
something similar to the event abstraction branch under the common API.
Assume that it's the integrators' or packagers' responsibility to either
use a recent library or a patched older library for sigrok builds.

All other non-Windows platforms remain unaffected, the presence check
never was fatal there. The data type check itself needs to remain in
place because the usb.c source code adjusts to its presence.