We are using "LIBFOO" prefixes for `LIBFOO_INCLUDE_DIRS` etc, and
therefore need to use this for the PREFIX with LibFindMacros also, so
that `"${${PREFIX}_FIND_VERSION}"` gets handled there properly.
The alternative would be to either manually handle/set the upper/mixed case
variants of those variable additionally, which is not as easy.
Keeping the existing names is useful for packagers.
Before this the version requirements with
`find_package(Unibilium 2.0 REQUIRED)` was not handled (a49cf5126), and
it prepares for using a required version with libvterm
(initially/wrongly ported in 1896c72a5).
