win32: bump WINVER to 0x0600
win32: bump WINVER to 0x0600

No point staying with XP.

Mentioned in pgbouncer/pgbouncer#100

markokr/libusual,ifduyue/libusual,markokr/libusual,markokr/libusual,ifduyue/libusual,ifduyue/libusual,markokr/libusual,ifduyue/libusual