configure: rename --enable/disable-libquvi to --enable/disable-libquvi4

--disable-libquvi creates the impression that it disables libquvi 0.9
as well. It doesn't, because it refers to libquvi 0.4, and 0.4 and 0.9
are practically completely different libraries. Make this more explicit
by renaming the switch to include the "4" version number.