Don't run extconf.rb while install of win32 binary gem, since this enforces DevKit installation.
Bump VERSION to 0.3.2
Use LIBPREFIX to determine naming of libusb dynamic library.

This should fix loading of the cygusb-1.0.dll on cygwin.
Fix return type of libusb_get_device_list().

Thanks to Paul Kunysch for the bug report.